Yes, of course I saw the season finale. And yes, I would agree that Negan is the show's best villain so far. He is thoroughly evil without being comically over-the-top evil.
But who did Negan kill in the season finale?
The closing dialogue indicates that he didn't kill either Rick or Carl. So we can rule them out.
What about Daryl? Negan hates Daryl, or course. However, there are larger considerations here, from a creative standpoint. Daryl is too integral to the show, too much a part of the show's "brand", if you will. If they kill Daryl, there will be a Walking Dead fan revolt. So probably not Daryl.
Well, what about Glenn, then? Glenn is a possibility, but I personally hope he wasn't the one; and I don't think he was. Glenn is among The Walking Dead's most likable, well-developed characters. He would be a real loss to the character dynamics of the show.
Carol is another possibility. However, the brutal, bludgeoning murder of a (non-zombie) woman would be boundary-pusher, even for a show as violent as The Walking Dead. I don't think Carol will be the one. Nor will Rosita or Michonne be the chosen.
I vote for either Abraham or Eugene.
Why? Several reasons.
First of all, both Abraham and Eugene were connected to an old plot line that has long since been resolved--that whole travel-to-Washington-DC thing. From a plotting perspective, both Abraham and Eugene are expendable.
Both are also somewhat one-dimensional characters. Eugene is a stereotypical wimp. (Admit it, you've wanted to box Eugene's ears multiple times since he joined the show.)
Abraham, a textbook foil to Eugene, is an annoying blowhard. Yes, I know he has a good side. That doesn't make him less annoying.
Either Abraham or Eugene could leave The Walking Dead without making the show less appealing. Fans wouldn't miss either one very much.
I don't have any inside information, of course; but either Eugene or Abraham would be my best answer to the question: Who did Negan kill?