Holy Angels can appear as men to help us on earth.
true
I wonder if the bad ones also appear in order to harass us, deceive us and pull us away from God etc.
true.
In fact Satan can appear as Christ and his fellow fallen angels can appear as angels coming to earth with "Christ" who is in fact a false Christ - Satan "as a being of light" - Satan appearing as Christ.
2 Cor 11:12 And I will keep on doing what I am doing in order to cut the ground from under those who want an opportunity to be considered equal with us in the things they boast about. 13 For such people are false apostles, deceitful workers, masquerading as apostles of Christ. 14 And no wonder,
for Satan himself masquerades as an angel of light. 15 It is not surprising, then, if his servants also masquerade as servants of righteousness.
2 Thess 2:3 Let
no one in any way deceive you, for
it will not come unless the apostasy comes first, and the man of lawlessness is revealed, the son of destruction, 4 who opposes and exalts himself above every so-called god or object of worship, so that he takes his seat in the
temple of God, displaying himself as being God. 5 Do you not remember that while I was still with you, I was telling you these things? 6 And you know what restrains him now, so that in his time he will be revealed. 7 For the mystery of lawlessness is already at work; only he who now restrains
will do so until he is taken out of the way. 8 Then that lawless one will be revealed whom the Lord will slay with the breath of His mouth and bring to an end by the appearance of His coming; 9
that is,
the one whose coming is in accord with the activity of Satan, with all power and signs and false wonders, 10 and with all the deception of wickedness for those who perish, because they did not receive the love of the truth so as to be saved. 11 For this reason God will send upon them a deluding influence so that they will believe what is false,
Matt 24:24 For
false Christs and false prophets will arise and will show great signs and wonders, so as to
mislead, if possible, even the elect
The ultimate false Christ - the one that could have a very good chance of deceiving even the very elect - is the one that comes as a bright and shiny Christ appearing in the heavens surrounded by a lot of bright and shining angels.
But as you also say - they can appear as normal humans or as Rev 16 says - they can appear as "frog-like" beings. To be "like frogs" does not mean they appear to BE frogs. Rather they appear to have some feature like a frog's feature in some aspect of their appearance.
Rev 16:13 And I saw
coming out of the mouth of the dragon and out of the mouth of the beast and out of the mouth of the false prophet, three
unclean spirits like frogs; 14 for they are spirits of demons, performing signs, which go out to the kings of the whole world, to gather them together for the war of the great day of God, the Almighty. 15 (“Behold, I am coming like a thief. Blessed is the one who stays awake and keeps his clothes, so that he will not walk about naked and men will not see his shame.”) 16 And they gathered them together to the place which in Hebrew is called Har-Magedon.