The third benefit of the Sentinel feat (PHB, p. 169-170) allows a melee attack when an enemy within 5 feet of you makes an attack against a target other than you (that doesn't have the feat).
If you kill this enemy with your attack, does their triggering attack still go through?
Same question if say you knock him back or prone, etc.
Best Answer
Because the feat doesn't specify a clear timing, your attack happens after the enemy's
Page 252 of the DMG has advice about Adjudicating Reaction Times. Specifically, it suggests following the timing specified by the reaction:
The reaction from sentinel doesn't specify that the attack interrupts the opponent, or that your attack occurs before their attack hits or misses. At best, this is unclear timing and at worst there is no timing specified at all. So, based on the advice in the DMG, the attack given by the feat would occur after the attack from the enemy. So, killing the enemy with the reaction attack won't stop their attack; their attack has already happened.
Similarly, if you were to shove or trip somebody with a reaction like the one granted by the feat, the reaction would occur after the enemy attack, and so tripping them or knocking them away wouldn't stop their attack either.
Of course, the DM does have license to rule otherwise
As is typical of 5e, this situation really comes down to how the DM feels about the specifics of the timing. If the DM were to disagree with my assessment, and felt that the reaction description does actually specify that it occurs before the enemy attack can hit, then they would be justified in using this same reference to make the opposite conclusion that I have. So, take my analysis with a grain of salt and consult your DM for the ruling that will apply to your game.