Rather than an "evil guy," why not a bounty hunter? You could take contracts from the Legion and the NCR, and any other faction.
I'd prefer being in the Legion, although being an actual Legionary is... difficult. I have a Centurion, with max melee, using a machete. I've recently started to put guns up on him, just because I know that it will be very, very, VERY difficult using only melee.
It's quite a misconception that Legionnaires only utilize melee weapons.
"The Legion excels at melee combat, as evinced by their choice of weaponry - primarily, the machete, which cripples limbs. They are not adverse, tactically speaking, to using firearms - scouts carry hunting rifles, while ranked Decanus may use a Cowboy Repeater. Ultimately, this leads to extremely effective Legion raids and surprise assaults. Legionaries are more versatile than the NCR troopers in that they are skilled with
both firearms and melee weapons, as well as having been trained to fight aggressively under any and all conditions, whether armed or not."