I wouldn't shoot an old friend…
…but stab him? Sure! It is possible shooting him too, but that's not as much fun.
I am of course talking about the final part of the “Bathhouse” mission in Splinter Cell Chaos Theory. It's a fine mission, very intense.
In Chaos Theory it's now possible failing softly, instead of getting an aborted mission if you kill a civilian you just get a reprimand over the radio, and a quite big deduction on the final score for the mission, 30% or so. There's also primary and secondary objectives and opportunity objectives. A score of 100% on a mission means you finished all objectives, never killed the wrong person, sucessfully hid all corpses and unconscious people and never got spotted. This is very hard to do, especially the first time through.
Another great thing is how the bad guys have much better weapons, a much better AI and they react fast, they don't spend time being surprised if they stumble upon you, they just shoot, faster than you can get away.
All surveillance-cameras in the game are bulletproof, but the pistol have a nice little universal jammer on secondary fire that can make any camera and lightbulb cease functioning for a while, 15–20 seconds or so, which is often enough. Speaking of weapons; the SC-20K assault rifle has been spiffed up visually, and it's more flexible now, for some missions it's possible to select a shotgun attachment instead of the launcher, or a 20 mm sniper rifle. Computers and retinal scanners can be hacked, so if cou have to kill a guard quickly instead of interrogate him and get the password all is not lost. It can be very hard hacking things.