In my last entry of Vengeful Musings I talked about how a person that is harmed may strongly desire for things to have turned out differently. Let us continue.
Even for as bad as B may want things to have gone differently with A, B understands on some level that it is quite impossible to change history. But that doesn't ease B's pain. Realizing this B is consumed with an overwhelming pain that can seemingly only be soothed by hurting A in some way.
The craving to hurt them back will not be easily quelled. It becomes a dark motivation for things that would otherwise be seen as quite fair minded and good ("the right thing for the wrong reason" effect). When this obsession has set in one thing is clear.
The mind has been poisoned.
B's mind has been poisoned with the hopes, dreams, and wantings of hurting A in way similar to the way they were hurt (although that "similar" can easily become "greater", overcompensation I suppose).
In the next entry I'll go into how this poison spreads.