My motivation here is to try make as certain as possible that he's NOT ESTj which I can't rule out as of yet, so I'm hoping someone will "prove" me wrong here.
Thing is, ESTjs are also known as "good soldiers". I certainly wouldn't rule out an ESTj serial killer and frankly I think there have been several. ESTjs tend to approach things from the point of one-upping, proving that something that something can be done. It's like a game. Killing the other person might not be construed by the psyche even as violence if the other person is someone who "is supposed to die".
I certainly have no trouble cutting people open in the surgery room, which is violence for a good purpose.
I guess I'd like to find something else, but the violence, to prove this guy was Se.
The tagging thing might be one. That smacks of vandalism, disrespect for property. Also he does look more like an ISTj. Thing is, I'd like more proof.