I don't agree with playing Professional by killing everyone. That goes against the meaning of the word Professional. I'm playing Blackwater Park right now, and there is no way I'm going to just kill people to get past. I can sacrifice my Suit Only challenge (which I've gotten in almost every level thus far), but I won't kill anyone other than my target.
EDIT: Got through Blackwater Park, Infiltrator/No Kills, after almost 2 hours only to find Cemetery Entrance is going to kick my ass for a while!
EDIT 2: Oh, the last level wasn't quite as bad, but I did kill 1 guard. I don't have a clue how to pass the first area without taking someone out. Ah well, I suppose that's a challenge for my Purist playthrough!