Issue is, according to the rules, the guard has defenses and hit points. So, even with a bonus from sneaking, you may miss; and even if you hit, it's possible you don't knock his hit points down the whole way, meaning he can then just yell out.
I'm of two minds on this:
Handle it as a skill challenge. Sneaky Sam, roll Stealth to approach and grab him; Burly Bob, make an Athletics check to pummel him good; Wizard William, test Investigate to make sure you don't leave any clues behind when you move the body. Failure on one of these tests introduces a complication, failure on all three means you dun goof'd.
Handle it as regular combat. Everyone roll to sneak to see if you get a surprise round, then make your attacks. See if you can knock the guard down before he cries out. After all, hit points exist as a measure of the guard's capability to defend himself, and part of D&D is portraying a quasi-simulationist world. The PCs may not be able to KO a well-trained guard right now, but once they level up, they will.