What is hard to understand??? Officers routinely take security jobs when away from their real job. Especially at apt complexes because they usually get discounted rent for providing that service.
If you were patrolling and saw an apartment in disarray, would you not go in and try to check the welfare of the tenant? What was the tenant on that made him so crazy is a better question.
No, I would not go in, even if the door was open. And if I did and a scuffle broke out, I would try to get back out, not fight back and eventually pull my gun and kill someone.