Depends, if they either realize that they wronged the person or agree with it or if the wronged person likewise, figure out that they maybe weren't wronged or they themselves were mistaken, then it can happen.
Since you are from Norway, you know about Breikvik, which is clearly a person feeling that he or in his opinion, Norway, had been wronged by whatever. So he was blaming others for this and decided to act on it. Now im not really convinced that he learned anything from it, I would assume that he is still convinced that he were in the right and everyone else just doesn't know it yet. Obviously only the minority will go as far as he did, but in theory I think its the same, whether its political motivated, religious or some personal motivation, some people can go very far if they feel wronged.
Even when its smaller things, like some people that might be angry at each other for years because of something which appear stupid to others, neighbors that can't get along and feel the others have wronged them. Point being, that a lot of people doesn't really reflect over these things, if they feel wronged. Because they are certain that it is the other parts fault.