6 And now you know what is holding him back, so that he may be revealed at the proper time. 7 For the secret power of lawlessness is already at work; but the one who now holds it back will continue to do so till he is taken out of the way. 8 And then the lawless one will be revealed, whom the Lord Jesus will overthrow with the breath of his mouth and destroy by the splendor of his coming. 9 The coming of the lawless one will be in accordance with how Satan works. He will use all sorts of displays of power through signs and wonders that serve the lie, 10 and all the ways that wickedness deceives those who are perishing. They perish because they refused to love the truth and so be saved. (2 Thessalonians 2:6-10)
I think this is very important in light of this verse:
13 Cain said to the Lord, “My punishment is more than I can bear. 14 Today you are driving me from the land, and I will be hidden from your presence; I will be a restless wanderer on the earth, and whoever finds me will kill me.”
15 But the Lord said to him, “Not so; anyone who kills Cain will suffer vengeance seven times over.” Then the Lord put a mark on Cain so that no one who found him would kill him. (Genesis 4:13-15)
This is one of those moments in the Old Testament where it seems that the more ancient the time period, the more knowledge that men seemed to intuitively possess about the Whole of Creation (heaven and earth, and not just the latter side of the assumed Hebraism).
Cain says that he would be "hidden from YHWH's presence", and seems to know that this means that his life would be one of almost totally unrestrained evil, as the Spirit of YHWHW (the Holy Spirit/Spirit of Christ/etc) has a major role in restraining wickedness in this world, which is a quite unimaginable mode of living. Outside of the presence of YHWH, and thus the presence of the Spirit, this kind of curse would make you something like a person stranded in the vastness of the ocean - particularly in the absence of a ship. It would be Hell, before actually being in Hell.
So it seems to me that the mark/sign/warning/omen is given as an act of grace, a stand-in for the Spirit as Restrainer that is specifically geared towards one category of wickedness as opposed to sin-in-general. It also, ironically, and YHWH does seem to display a very sharp sense of irony quite often, prevents Cain from having an easy out from the curse. So it's justice and justice (grace) and justice. And that's why the mark was both given and necessary. Not really to prevent a revenge killing, but to provide a morsel of the common grace, if you want to call it that, which prevents life of Earth from being a total living Hell. Because that is what Cain's life would have been otherwise.