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Some things to keep in mind here:
- All rights come from God. All humanity are the subjects, he is the King.
- The Canaanites, who he commanded the Israelites to "cut up" as you put it, were heavily involved in human sacrifice, even sacrificing children (look up Ras Shamra sometime). They had been a wicked people for longer than 400 years, the time in which Israel had spent its time in Egypt (Genesis 15:12-16). In short, they deserved it.
- The plagues were a direct attack on Pharoah's supremacy and authority, as well as the notion that Egypt's gods were anything of significance. Furthermore, remember that Pharoah ordered the slaughter of all Hebrew boys.
- Pharaoh according to Ex ordered the two Hebrew midwives to kill the boy babies. This they did not do but gave the excuse that the Hebrew women birth too quickly and that seemed good enough. Pharaoh did not seem interested in taking more effective action.
- 'All rights come from God'
OK, if you believe God exists, and you believe he is the potter and we are the pots and he can break any he chooses, well if he wants to kill someone he can do it himself.
In any case this leads to the other problem that if Satan is a killer why does God do over 99% of all the killing in the Bible?
- The Canaanites...
This is where a commentary might come in useful, except an evangelical commentary will only give information to make scripture sound true and nothing against it, so maybe not.
Human sacrifice was mainly to placate god to ensure the rains came when needed, agricultural societies would be far more prone to this than a shepherding society like Israel.
But despite the lack of perceived need it still went on in Israel.
Lev 18: 21 Do not give any of your children to be sacrificed to Molech, for you must not profane the name of your God. I am the LORD.
1 Kings 11:7
On a hill east of Jerusalem, Solomon built a high place for Chemosh the detestable god of Moab, and for Molech the detestable god of the Ammonites.
2 Kings 23:10
He desecrated Topheth, which was in the Valley of Ben Hinnom, so no one could use it to sacrifice his son or daughter in the fire to Molech
Judges 11:31 whatever comes out of the door of my house to meet me when I return in triumph from the Ammonites will be the LORD's, and I will sacrifice it as a burnt offering."
32 Then Jephthah went over to fight the Ammonites, and the LORD gave them into his hands. 33 He devastated twenty towns from Aroer to the vicinity of Minnith, as far as Abel Keramim. Thus Israel subdued Ammon.
34 When Jephthah returned to his home in Mizpah, who should come out to meet him but his daughter, dancing to the sound of tambourines! She was an only child. Except for her he had neither son nor daughter. 35 When he saw her, he tore his clothes and cried, "Oh! My daughter! You have made me miserable and wretched, because I have made a vow to the LORD that I cannot break."
36 "My father," she replied, "you have given your word to the LORD. Do to me just as you promised, now that the LORD has avenged you of your enemies, the Ammonites. 37 But grant me this one request," she said. "Give me two months to roam the hills and weep with my friends, because I will never marry."
38 "You may go," he said. And he let her go for two months. She and the girls went into the hills and wept because she would never marry. 39 After the two months, she returned to her father and he did to her as he had vowed. And she was a virgin.
Despite my Bible study course jumping all over the place, some of it did get sifted out in my mind.
Basically in the earlier parts of the O.T. the Israelites did all sorts of really nasty things like the odd bit of human sacrifice. In the gospels although they did tithe their herbs they also got up to even worse things like saying their prayers aloud at the street corners, and for that the nation of Israel was totally destroyed.