Impostors have a kill cool down (we play with 25 seconds usually), which is what stops them from just murdering everyone. Once their cool down is off, they can just slice and dice someone, ideally when nobody else is looking. Here’s an example of me sniping Zlubars, and I even knocked out the lights via sabotage first.
Killing is the most dangerous thing you can do as impostor, both for yourself and for your poor victim. Still, you gotta kill to win, so timing them is key. They are helped (as in that clip) via sabotage.