while its generally true that the military prefers to keep things short and simple (why they have acronyms for even the simplest things), i do think it would be funny to call him Psycho Tree Killer (slaughterer, murderer, burner, etc), just to get on Gunty's nerves, and then when theyre not talking to him, just about him, call him PTK... 

...Or... something that goes with PTSD, like Psycho Tree-Sizzling Dude XD... or Tree-scorching dude