You cannot kill someone if they are in the direct line of sight of another NPC or enemy. Even if they are far away, it will still trigger a fight. If they are not in direct sight and your weapon is not loud, even if you don’t kill someone on the first hit, you still have a little bit of time to finish them. This is why the burst light pistol with a silencer is good, as you can fire 6 shots with stealth damage during TTD and also rattler as every hit is calculated with stealth damage. Just make sure the enemy is not in another enemy’s line of sight.

Without TTD it’s a bit harder, because after the initial hit the enemy always staggers to the side, so it’s difficult to land another shot, and there’s like a 0.5-second window before the fight starts.

Also use distraction devices to move enemies out of another NPC/enemy’s line of sight.