I think there is a difference between being hyped up, which is what it sounds like you are describing and a modification of how aggressive/violent a person is. I’m not disputing that it’s possible when dealing with basal emotions like this, that you can become absorbed in it and be drawn to act out of character. I’m disputing that engaging violent media modifies a persons levels of or attitude towards aggression/violence over the long term. I’d describe your foray into whatever media you were exploring one of immense value. From it you learned how you can become fired up, and how to control and regulate those emotions. Hence my strong opposition to people following the typical line of, “Violence is bad, so consuming media which shows it, is bad.”
There may be some who seek violent media for unhealthy reasons and/or be harmful to society because of their attitudes towards it, but I don’t accept that engaging in violent fantasies is unhealthy in itself. To my understanding those who commit acts of gross violence do go through desensitisation, but they will do so regardless of the availability of violent media and I don’t believe are stimulated to do so because of exposure to it; at least not when they engage it knowing it is a fantasy and not reality. The reasons for following that road are seated much deeper, and involve their perspective of self, others and the world around them.
Shifting a little off topic but, if as an authority figure you take the stance that you don’t approve of violent media and discourage those you are responsible for from viewing it, you will miss the opportunity to monitor their reaction to that media when they seek it out of your range of influence. This only further strengthens my resolve to disagree with the attitude that violent fantasies should be avoided or discouraged.
I believe as Leijona describes above, that the cause of violent behaviour comes from elsewhere in the psyche. I also believe that encouraging abstention from violent media or fantasies is only a harmful thing. Those who find it distasteful, already are disinclined to do it: the discouragement is moot. Those who don’t find it distasteful enough to avoid, or who are curious, may come to understand a part of themselves and create a healthy outlet for it. Which otherwise could be turned to something destructive and harmful, to themselves and/or society in general.
If you are inclined to go on a senseless and unexplained killing spree in a fantasy situation, I’d say not to try to constrain yourself, but to put your effort into thinking about why. Actually doing it in a fantasy could well give you even more information about that. You are far more likely, in my opinion, to learn something deeply useful about yourself than you are to degenerate into a violent person in waking life.