Somebody who hires himself out to kill for money? Obviously mentally ill.You might say that the soldiers were just doing a job. That is true. But what about a hitman or a hired assassin (which, I guess, are kinda the same thing)? Are they are just doing a job or are they mentally ill as well?
But wouldn't, under your definition, a government assassin who's job it is to take out assigned targets then also be mentally ill?