Read Romans 12:17-21, "[17] Repay no one evil for evil, but give thought to do what is honorable in the sight of all. [18] If possible, so far as it depends on you, live peaceably with all. [19] Beloved, never avenge yourselves, but leave it to the wrath of God, for it is written, “Vengeance is mine, I will repay, says the Lord.” [20] To the contrary, “if your enemy is hungry, feed him; if he is thirsty, give him something to drink; for by so doing you will heap burning coals on his head.” [21] Do not be overcome by evil, but overcome evil with good" (ESV).
Notice the logic, in particular the transition in verse 19. Paul explains that the reason we can love our enemies (v. 17-19a) is because God promises to avenge (v. 19b). One of the reasons we strike back at people is because we feel indignant at the perceived injustice. If we don't do something, they will just get away with it. Therefore, we want to set things right.
Here, we are reminded that every sin, every injustice, will be punished....whether in hell or at the cross. Every single one in the universe.
The is why we can return evil with good. We don't feel the desperate compulsion to fight back, to defend our name, when we know God will keep his promise and uphold His honor.
