An activity or phenomena that many young males go out of their way to achieve in order to obtain emotional equilibrium and relieve either boredom or sadness. It is often achieved through the use of video games such as "Call of Duty" since the physical act of killing someone or something is often frowned upon. The act of "killing some shit" may also be used therapeutically.
He just got dumped, he must feel terrible. He should go and kill some shit that will make him feel better.