Sigh... Not another..
This game is not Team Fortress Two sir, there isnt no backstabbing or crab walking here. The main goal of being able to disguise is to get a point of view from their perspective and exploit them, for example, you can take out their mines, their snipers, hell you can even throw a sticky and then run away.
Calm down, son. There isn't a reason why you shouldn't stab people in the back if they're alone. I mean, I guess you can leave that sniper that's just sitting there headshotting your teammates from across the map, nothings stopping you from not killing him, but I'd grow some balls and kill him because I don't like it when people kill my friends.
Also, I've always felt that stickies were useless. Put it on a guy, and he runs away to his friends, who hit it off. At least let the sticky detonate after it gets hit off...