"Wherefore if ye be dead with Christ from the rudiments of the world, why, as though living in the world, are ye subject to ordinances." (Col 2:20) "For the wisdom of this world is foolishness with God. For it is written, He taketh the wise in their own craftiness." (1 Cor 3:19) "Any technique however worthy and desirable, becomes a disease when the mind is obsessed with it." (Bruce Lee, Tao of Jeet Kun Do)
Your mind is likely made up, but I'll answer in case it's not. I'll focus on the "killing our opponents" and "cause serious injury" part, i.e. violence and resistance. Arguing that martial arts is violence, offensive or defensive.
2 camps are contrasted, who and what God loves and what he hates: "The LORD trieth the righteous: but the wicked and
him that loveth violence (hamas חָמָס)
his soul hateth." (Ps 11:5) "Deliver me, O LORD, from the evil man: preserve me from the
violent (hamas) man;" (Ps 140:1) Equivalating "evil man" with "violent man". The Lord said, "Blessed are the peacemakers: for they shall be called the children of God." (Mt 5:9)
hamas (חָמָס) is defined as "violence and wrong doing, specifically physical violence, but also injurious language, harsh treatment, etc... (and relates) in general of (to the) rude wickedness of men, their noisy, wild, ruthlessness".
H2555 - ḥāmās - Strong's Hebrew Lexicon (kjv).
A side note: Kung Fu (功夫) refers to
any study, learning, or practice that requires patience, energy, and time to complete. It can refer to any discipline or skill achieved through hard work and practice, not necessarily martial arts. Anything can be "kung fu" (even studying Scripture), hence the philosophical dimension. Whereas Krav maga: k-r-v (קרב) n-g-a (נגע) = combat/battle + contact/touch. From what little I know, is purely physical system and various MMA designed for lethal violent force, namely against enemies of State of Israel, e.g. executing Nazi war criminals, against ISIS etc.
But the Lord says in Matthew 5:21-22 "Ye have heard that it was said by them of old time,
Thou shalt not kill; and
whosoever shall kill shall be in danger of the judgment. But I say unto you, That
whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca, shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of hell fire." This could ofc be interpreted as non-lethal force is permissible for self-defence. But that would be a very fine line. One would have to use violence for self-defence or good reason, without any trace of ill intent, without anger or emotion, robotic movement. Justified homicide, pure and legal intent. Very difficult.
Read in light of the same line of Christ's teaching on "adultery", as reference to the entire decalogue, it teaches us that action and intention are the same. Both actual harm and thoughts of harming others are the same sin in the eyes of God. Hence Cain and Abel. Esau and Jacob. Herodians who executed Christ vs the "Meek and humble (who) shall inherit the earth." The violent party is the criminal group.
Reference to violence as opposed to
instruction to violence is the key to unlocking your question. Paul famously spoke in local terms, embodying local culture, "to the Greek I became a Greek" including Greek-speaking Jewish settlements. People familiar with a culture of
gymnasium (weight lifting and body building), the culture of
olympics (athletics), unbelieving pagan Greek philosophers, and if you study his work you'll see it done everywhere as a tool for comparison. He credits them but weighs the value of their beliefs (pros and cons) to point men to the Gospel, to God's truth, not to in support of paganism or pagan values. To "run the race" to salvation, not an actual marathon to be honoured among men (which is actually "abomination" to God, Lk 16:15). See,
Did Paul quote pagan philosophers?
Temporal vs eternal value. Manpower vs Divine power. The reference to "bodily exercise" (
somatikos gymnasia) and famously many other major Greek philosophies in Paul's work was out of respect and acknowledgement, to point the locals to a greater and higher purpose (holiness, godliness). "But godliness is profitable unto
all things (πάντα,
panta)" having promise of the life that
now is, and of that which is
to come."
Proof texts against violence are both implicit and explicit.
As mentioned, Cain vs Able is the earliest case. Whilst it's true people sometimes have no other option, violence is certainly not the rule, not encouraged. No rabbinic school or church will worship while kicking ass. Military service would be an exemption. E.g. Saul enlisting civilian shepherd boy David as champion against Goliath.
When David cut Saul's robe in 1 Sam 24, the accusation was that "
David's heart smote him". Commentators teach it was wrong. Firstly, touching the king, but also the act of violence (or threat of violence) against any brother or creature of God (not our property). Brotherly love, world union under God is his will.
David's own defence emphasised the innocence of his
action but nothing is said about his
intention (i.e. pride, dishonour, rebellion, treason). He denies wrong doing, even justifies his actions by comparing himself to the King, pleading self-defence. "
I cut off the skirt of thy robe, and killed thee not, know thou and see that there is
neither evil nor transgression in mine hand, and I have not sinned against thee; yet
thou huntest my soul to take it." But David was anointed, God's chosen. Whereas, Korah and his rebel army were a similar case, but were not chosen, going against God's appointed leadership, and the rebel para-military was consumed, all dead.
The 6th commandment is blunt: "Thou shalt not kill" (Ex 20:13). It's the first commandment directed at loving neighbourly behaviour (with 1-5 for loving God). Lethal martial arts practices would illegal, in violation. Non-lethal would be legal, but only in 'the letter of the law', not 'the spirit of the law'. The command lacks an
object (or pronoun him, her, it) thus implies that we are to not only to not kill/destroy but embrace everyone of God's creatures. As Adam. Rom 12:18 says "If it be possible, as much as lieth in you,
live peaceably with all men." For all intents and purposes, peace is the goal. Inclusivity, diplomacy. Noah's ark vs God's destruction. To be fishers of men, saving God's creatures from destruction.
Isaiah 60:18 - Violence shall no more be heard in thy land, wasting nor destruction within thy borders; but thou shalt call thy walls Salvation, and thy gates Praise.
Whether violence as a means of peace (e.g. self-defence)
justifies violence on the whole, that's up to the individual, and there's a toll. Can you afford it?
Injury or destruction is a matter of damage to God's property (his creation). Be it premeditated or unpremeditated violence, the heart of the issue is the act of "killing" itself (anything, especially someone's property or possession: house, wife, animal, son, servant, etc, per the 10th commandment). Whether homicide or theriocide (animal slaughter): "Death at the hands of any man (comp. Gen. iv. 14),
man and beast being included in the same statute (ib. ix. 5, 6)."
HOMICIDE - JewishEncyclopedia.com. "Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the Lord set a mark upon Cain, lest any finding him should kill him." (Gen 4:15)
We are told of spiritual war, in heaven and on earth. Suffering in patience is part of faith, training endurance and inner strength, prayer. Strength in weakness: "Therefore I
take pleasure in infirmities, in reproaches, in necessities, in persecutions, in distresses for Christ's sake: for
when I am weak, then am I strong." (2 Cor 12:10) To be dead to the world and alive in Christ.
"The Lord is a
man of war" (Ex 15:3, and many others) And Christ said "Think not that I am come to send peace on earth: I came not to send peace, but a sword." (Mt 10) "Dearly beloved, avenge not yourselves, but rather give place unto wrath: for it is written,
Vengeance is mine; I will repay, saith the Lord." (Rom 12:19) "Say not, I will do so to him as he hath done to me: I will render to the man according to his work." (Pro 24:29) "Say not thou, I will recompense evil; but
wait on the Lord, and he shall save thee." (Pro 20:22) "But I say unto you, That ye resist not evil: but whosoever shall smite thee on thy right cheek,
turn to him the other also." (Mt 5:39) "And unto him that smiteth thee on the one cheek
offer also the other; and him that taketh away thy cloak forbid not to take thy coat also." (Lk 6:29) "Likewise, ye younger,
submit yourselves unto the elder. Yea, all of you
be subject one to another, and be clothed with humility: for God resisteth the proud, and giveth grace to the humble." (1 Pet 5:5)