You seem to have encountered Rampage, a weapons dealer who is a member of S.K.A.R. (Soldiers of Kaos, Anarchy, and Ruin), he seems to be getting stressed and you can see that he is turning into a freakish green monster underneath the suit of advanced armor he is wearing.

"WHAT THE HECK DO YOU WANT!!" he says angrily, growling in frustration since he's annoyed by who you are.