Sounds like a nice dilemma for a character on a TV show: do I reveal that I’m an alien tasked with wiping out all human life, or do I keep quiet about the ‘alien’ thing and simply take the rap for the guy whose appearance I stole?
If he goes with the first choice, he’s practically guaranteed a successful insanity defense. Life in a mental-care lockup might not be so bad for alien-Harry: I bet they have Law and Order episodes on all the time in their communal areas.
In any case: there are so many ways the writers could go with this show. It will be hard to wait to see what they’ve decided!