A study by Finnish researchers published in the journal Emotion concludes that players of violent video games experience relief when they are killed in game, and that repeated play does not desensitize players to violence:
From the article: "instead of joy resulting from victory and success, wounding and killing the opponent elicited anxiety, anger, or both." In addition, "death of the player's own character…appear[s] to increase some aspects of positive emotion." This latter finding the authors believe may result from the temporary "relief from engagement" brought about by character death. Whatever the underlying basis, however, the results seem highly counterintuitive…
The researchers also found that: 1) Players showed no signs of desensitization over the course of multiple play sessions; and 2) Subjects who tested higher for psychoticism (based on a pre-trial psychoticism questionnaire) experienced less anxiety from killing enemies. That higher psychoticism would correlate with lower negative feelings about violence is not surprising. It is interesting, however, that players showed no signs of physiological or emotional desensitization. While this doesn't necessarily disprove that desensitization to videogame violence can occur over long periods of time, it does suggest that brief exposure has little or no desensitizing effect.
(via Collision Detection)