Hypothesis: If you buy a punching bag, or release your aggression by playing Quake, or by screaming, then you will be less violent and aggressive in day-to-day life, having "released" your aggression.
Deprecated? No, disproved!
What actually happens is that the person's inhibition against violence lowers and they become more likely to engage in violence against everyone, even innocent bystanders. Violence begets violence.
Not only has the catharsis hypothesis not a skerrick of empirical evidence in its favour, but there are studies showing the exact opposite.
I know that lots of people use this argument to explain poor behavior including:
These cases are different from the opening example, because the actor is not releasing stress on a displacement object (punching bag, Quake monster, etc)< but upon another real person.