As they mentioned above, rolls are not always needed to determine something. If a PC has a guy tied to a chair, and he says, "I pull out my gun and just shoot them in the head". Maybe it is just as simple as saying, Bang, the guy is dead. If the target of the aggression has no realistic way to avoid getting shot in the head, then that is that. If other people are trying to stop it or might interfere, maybe its an acting under fire instead.
Now, you can lead up to the moment where a sleeping guy gets offed by making the situation have other complications. Say you've got a player sneaking into a enemy bar to assassinate some guy. If they can just walk in, maybe they notice other people around the bar that might become a problem after a fight breaks out; maybe they notice the target hanging out with his kid; or maybe everyone inside will damn kill the PC on the spot if he's seen. Each of these examples sets up a different kind of tension, and you can probably think of dozens of ways to make a scene complicated.
It is these complications that make for a gripping scene. So your guy have to sneak in without anyone noticing who they are, and get all the way up the stairs and into a room to wait for the guy. Act under fire (risking being recognized) as you push your way through the crowd with your hoody down. The player gets into a hiding place and waits for the guy to come in and fall asleep. Is he alone, is there loud music playing? If so, maybe the PC can just step up and whack the guy, no questions asked. If its way to quiet, maybe moving on this old creaky floor makes a lot of noise, you tell the PC they can off the guy, if they can cross the room to him without waking him/ whoever else might be there. Roll Acting Under fire (against alerting danger). If the Player hits then maybe they're just dead. On a partial, well maybe someone heard, the target woke up and screamed first, the other person in the room woke up but not in time, the player kills the guy and almost get out before someone notices, or they kill them and open the door looking into the eyes of their buddy. On a miss? pick your poison, but don't inflict harm on the PC without first announcing the harm and giving them a chance to react.
Weaknesses on vehicles, keywords on weapons, complications in a scene aren't there to always get in the character's way. They're there so on partial hits and misses, you have a ready supply of things that could go wrong. Additionally, they provide the tension and suspense that makes doing the task perfectly all the more rewarding AND it also paints the risks to the PC ahead of time so they're not totally surprised/taken-a-back/thinking you screwed them when the thing that goes wrong goes wrong.
Just as a tip for AW combat, Armor is not an unpenetrable shield either. If the knife cant hurt that two armor having guard, maybe they have to get way to close, risking harm, detection, initiative before they can sink that knife in-between a crack. You have the right to tell them to act under fire while they attempt to seize the target by force, the seize is just to land the blow, the act under fire of the armor's protection is to see if the knife will stick in them all harmless like or actually end the guy. Likewise you can use missed rolls to expose vulnerabilities in armor as well.
As for how enemies ACT, think of it this way. When a PC hits on a 10+, it's like the enemy misses. So always describe the enemy action and keep it right there in front of the player where they can see, just when it comes down to decide the results, let the player's roll decide. This is why when a PC misses, one of the more common ways to handle the situation is to reverse the move. I'll provide a quick example:
Joe (pc) has a big damn knife and is approaching Leer (npc). Joe is all pissed over something Leer just did--seeing him coming, Leer pulls out a handgun and rests it on the counter--tapping his finger on the barrel. Maybe words are had, maybe not, but whatever the case, Joe's player says he bum-rushes the counter, aiming to leap over and butcher Leer like the pig he is. You say sure, roll seize by force. Joe rolls a 4, Miss. What do you do?
Well, its pretty clear from the roll that Leer is going to pull up that gun and start shooting Joe as he closes in. So you CAN (but don't have to) consider the situation as if Leer just made the seize by force roll on a charging Joe. Look at the seize by force and pick some options, deal more harm, suffer less, taking definite hold of his position. Deal harm+1 to Joe, describe joe's attacks as dealing harm but not enough, describe it all cool like, but end the action with something like Leer wounded from a damn knife wound (not fatal), and Joe having been shot a few times maybe having been forced into some bad spot taking cover from the guns. Of course, you'll want to up the anty in my opinion too, so maybe Joe also hears the sounds of more coming to the gunfire. He doesn't know whose side they're on. What does he do now?