Painkiller / Painkiller Black
There is a hud transparency option in the menus, but that only adjusts the transparency of the background around the numbers on your hud, not the entire hud transparency. At this time, there is no known way to actually disable the hud in Painkiller, but you can change it's size to be so small that the game no longer renders it.
To do that, edit the config.ini file in your painkiller/bin/ directory.
Change the following line to this variable:
Cfg.hudsize = .00001
the game may change it to:
Cfg.hudsize = 1e-005
Either will work.
Save the file, start the game, and enjoy staking enemies without that ugly hud.