If I have my Harrenhal (Stark) in play, I am first player, and my opponent marshals Melisandre. I kneel my faction card and kill Melisandre with Harrenhal. Does my opponent get to do Melisandre's reaction.

We are uncertain about if the reaction gets a chance to resolve, because she is killed, but she still technically entered play.

I want to say yes, but I would like some clarity to be sure.

Moribund doesn't exist anymore; there is no staying-in-play-until-all-Reactions-are-resolved. By the time it gets to his turn, she's dead and gone.

Thank you, Grimwalker!