The secret to a peaceful life that is free of negative thoughts is apparently not thinking about God of War and its ilk.
It seems it's not just playing violent games that affects your behavior; psychologists have found that thinking about playing them can make you more aggressive as well. A study conducted by Dr. Bryan Gibson of Central Michigan University and Dr. Brad Bushman of Ohio State University showed that men who played violent games and then thought about them afterwards showed heightened levels of aggression as long as 24 hours later.
The researchers randomly sorted 126 college students, split roughly evenly into males and females, into six groups and had each group play a different game. Half of the games were violent, such as Mortal Kombat vs. DC Universe, and half were non-violent, like Guitar Hero. Each subject played his or her designated game for twenty minutes, and then half of the participants were asked to spend the next 24 hours thinking about the game and how they might improve the next time they played, while the other half were given no specific instructions.
The subjects returned the next day and took part in a competitive reaction test against an opponent of the same sex, were the winner was able to punish the loser with painfully loud sounds, ranging from 60 decibels to 105 decibels. The researchers also included a silent, non-aggressive option. The test showed that the men who had played the violent games and thought about them were more aggressive than those who hadn't been asked to think about the games, or had played non-violent games. Interestingly, women were not found to be more aggressive at all, regardless of which game they played or whether or not they thought about them.
The results are interesting, but it seems pretty likely you'd get similar results asking the subjects to think about a fight they had with a significant other, or the person who cut them off in traffic, or even a football game they'd watched. You can read more about the study here.