Shoulda, coulda, woulda. We have no idea yet as to the exact details in how he ended up there. I seriously doubt his co-worker called him up and said; "My boyfriend just abused me and trashed the place, can you come over and shoot him for me?"
Sometimes these things have a way of escalating far more rapidly than one realizes.
Maybe police had already been called, maybe not.
It may be that boyfriend left, and she called her co-worker, Heng to come pick her up and she was trying to get packed and out and the boyfriend barged his way in.
Or maybe she wanted him locked out and "took a stand", which then I agree is a pretty stupid move on the part of Heng to get involved.
Or maybe Heng really is just some psycho, or a George Zimmerman type walking around with a paranoia, or chip on his shoulder, just looking to kill someone. Time will tell.
In the meantime I'll give him the benefit of the doubt. Especially because he has no record and the dead aggressor does have one.
