I really love the detail you put into the character in this one.
Some people have mentioned the gun issue, but for me the problem is with his boot (and the kneepad above it), the left one particularly feels off because everything else is pretty much detailed, save for that one, and it kind of breaks what otherwise would be a pretty good, detailed character image here.
The detail you put into him looks fantastic, though, the only thing that's bothering me is his body language. Especially his lower body, it doesn't exactly "connect" with his upper half and isn't as menacing. It looks like you were trying to show movement from his knife slicing, but I'm not sure that came through very well. Maybe it's just me. Otherwise, the time you put into this definitely shows!
The picture looks great and the outfit has a lot of detail. Also the colourscheme works out great! I feel the position of the Arm is strange though (more precise the hand-arm position). I'm not a saint to anatomie, but try it out for yourself, its really stressful to turn the lower fist away from elbow direction. The blade could be turned more towards the viewer or the arm could be lower I suppose.
The character model is decent, but that weapon is ugly as fuck. My tip for you is to study 'weapon anatomy' and get some better ideas for weapon designs. Everything else is good, though. The character as I've previously mentioned, the environment and the shadows. I think the rest of the picture is good, but that weapon is just terrible looking.