What is the name of this fallacy? Someones says that because everyone cheats, I also can cheat! Or because someone stole my money, I got penniless and had to stole from someone else!
I would call this is a "two wrongs make a right" fallacy. From the Wikipedia article: "Two wrongs make a right" has been considered as a fallacy of relevance, in which an allegation of wrongdoing is countered with a similar allegation.