There’s an episode of Danger Man (“Time to Kill”, I think) where John Drake is sent on a mission to assassinate an enemy agent - Drake voices his objections, but he takes on the job nonetheless. Probably doesn’t entirely count, though, since it turns into a straight fight as the situation develops.)
While I’m on “roles played by Patrick McGoohan”, there’s the “Hammer Into Anvil” episode of The Prisoner, in which Number Six comprehensively destroys a particularly evil Number Two - technically, the guy is still alive at the end of the episode, but he’s almost certainly doomed. And then there’s the conclusion of “Once Upon a Time”… though I suppose that one’s arguable, since it’s established that only one person can survive the Degree Absolute procedure (and they reanimate that Number Two anyway in the next episode. Assuming anything that takes place in that particular episode is real.)