2. The Punisher (Marvel comics, movies)
Frank Castle appeared in the Hero thread, but he's far more appropriately placed here. He's a murderer, a serial killer, though fortunately he only goes after the bad guys. Of course, exactly what constitutes a bad guy to this one man judge, jury, and executioner is exactly what makes him so feared by all. Drug dealers, mafiosos- but how about the guy who steals from the register at work?
3. The Man With No Name (Spaghetti Westerns)
This guy... well, he has no name. He roams the lawless territories of the west, seemingly only out for himself. He has a conscience, though, that makes itself manifest from time to time, and he has been known to ride to the rescue of the odd damsel in distress.
4. Snake Plissken (Escape From New York, Escape From L.A.)
This guy's survived so many near death scenarios that everyone is surprised to see him still around and kicking. A complete and utter self-serving bastard, he nevertheless usually ends up doing the right thing- if only because it's the exact opposite from what the people he works for (who are even more lowdown than he is) want him to do.
5. Solid Snake (Metal Gear series)
This guy's the best there is at what he does... and you all thought that was Wolverine's shtick.
A tough as nails, hardcore soldier, who has been used and abused by the government he works for time and again without asking questions. He's recently gone rogue, though, and is even more dangerous than ever.