I wouldn't count Niko out. Niko didn't get super rage pissed easily, but when he did he went hard. (remember, killing the girl he liked/his cousin caused him to take out most of the Alderney Mob all by himself)
It's not quite as clear cut as everyone makes it. Killing a suicidal meth head who allows you to clutch them in your arms isn't the same as killing a sociopathic ex-soldier with a history of solo-ing men who are prepared for him.