Killing Soldiers
There aren't as many lethal takedowns in the game as there are non-lethals, if only because the game generally seems to encourage you to keep as low a profile as possible.
Melee Kills
As with non-lethal takedowns, there are two kinds of lethal melee takedowns. The first involves approaching an enemy and pressing your primary attack button; if you get into the correct distance from the target, then Sam will use his knife in some sort of especially cool-looking fashion to instantly kill the enemy without him even getting the chance to utter a dying scream. If you use a lethal knife attack from behind, you can also hold down the button to automatically pick up your dead foe and move his body.
Alternately, if you first grab an opponent, then feel free to herd them somewhere nice and dark before hitting the primary fire button; this will send Sam's knee directly into your foe's spine, snapping it and killing the soldier instantly. Ouch.
Non-Melee Kills
Railing Kill!: Everyone's favorite B-movie action cliché is alive and well in Splinter Cell: Double Agent, but with a new twist. If you can get Sam to hang from the lower edge of a walkway and wait for an enemy to walk by, then you should get the Grab Character interaction choice, which will cause Sam to reach up, grab the foe, then fling them over the railing to their death at the bottom of wherever you're hanging. It's fun, but it can be difficult to get into position for this kill without being spotted. Also note that in some instances your victim will let out a yell before falling to their death, sometimes alerting nearby foes.
;)
Ice Kill: One of the new killing maneuvers in Double Agent is the ice kill. In some of the early levels, you’ll find yourself swimming along underneath ice packs. If you can get below a thin spot in the ice, you can grab any soldiers that pass by on top of the spot, pull them under, and stab them in the chest to finish them off. It’s a pretty sweet maneuver, if we do say so ourselves, although obviously breaking the ice will prevent you from performing it again in the same spot.
Note that when you’re underneath a weak spot, you can hit the whistle button (down on your d-pad) to turn over and bang on the ice, thus hopefully luring a soldier over to you if there isn’t anyone above you.
Ranged Kills
Well, duh. If you want to kill someone from a distance away, you're going to use your guns. The primary choices here are the silenced pistol, which is best used against stationary enemies that you can get close to, and the SC-20k Rifle.
If you can get right up next to an enemy, then your pistol should be able to drop them with a single shot to the head. The good thing about using the pistol as a killing tool is that it's really quiet; although it does make some sound, it generally won't alert any other soldiers unless they're standing quite close to your target. The bad things about it are its accuracy (you're going to need to be fairly close to hit your target; a missed headshot will almost certainly cause an alarm) and a fairly small ammo loadout for it. You're going to be using your pistol a lot to shoot out lights, which can cause you to run down ammo if you also use it for headshots.
;)
The scope for your rifle will also help when it comes to getting headshots. The benefit here is range, in that the rifle can pop off shots at a good distance away with decent accuracy, while the drawback is noise. Although the rifle is silenced, it's still noisier than the pistol, and will often alert enemies close to your targets, especially if you accidentally hold the button down longer than intended and shift into automatic fire mode. To use the scope, bust out your rifle, click the right analog stick, then hold the left trigger to hold your breath, which will further increase your aiming abilities.
Other than those, you also have the shotgun and sniper rifle attachments for the rifle. We found both of these to be a bit too noisy for our style of play, but if you want to run-and-gun and don't mind a few alarms here and there, then there's no reason not to use these powerful weapons.
Lastly, frag grenades can kill enemies at range, although it's difficult to do so. Most enemies will run as soon as they hear or see a frag bounding towards them, and the kill radius isn't substantial, so you'll usually wind up wasting frags unless you can chuck them at enemies in tight quarters.
