One of the fiat things is that a lot lies in the details.
Pushing a boulder off a 100' cliff on a sleeping giant may or may not kill them. . .
It's not instant to kill someone, in armor, snoozing at their guard post, by choking them. . . .
If you have the guy tied, unarmored, you're straddling his back, with your left fist tangled in his hair pulling his head back to offer lots of neck, and you have a two foot long, razor sharp knife held to the underside of his neck, yelling at his friends "One step closer and I kill him!"
A lot depends on the details, with the boulder I'd do a maneuver to see how well you did shifting the rock, with a fall/crush attack resolved after, with the choke I might allow a first grapple attack at massive bonuses, and likely a surprise bonus to initiative on the next round, but the guard can fight back. . .the tied prison is more like a "Make a Maneuver and don't fumble"