My thoughts exactly!!! Did the evil of the director or the place finally infect him??? I found his expression very confusing.

As some reviewers of the movie have pointed out, the final 20 minutes could've been cut out entirely. The grin feels like a bad attempt to provide an ambiguous ending. That being said, I loved this movie and wasn't sure whether the protagonist was to believed or not through the entire thing. I guess the entire thing is open to interpretation.

I finally did what my lazy self should have done in the first place : search this wonderful place that sometimes is the Internet.

Here's an interesting theory that actually makes sense if one acknowledges he's not become a white knight as we are led to believe, but quite the opposite. By now I don't remember the movie all that well so I can't tell how consistent it really is with the details.

"My thoughts on the smile are this. No, it is not the Baron who has somehow transcended onto Lockhart. He is shown to have no such powers. Lockhart desires success unlike no one else his age. It has consumed him to the extent that he’s willing to throw Pembroke under the bus to escape the fudging he has done. He hungry for money and power. He’s not a saint to start off with. He’s quite the evil corporate businessman in the making. Then he's made to go nuts by drinking eel water. He is now the only surviving heir to the knowledge of eternal life. Hannah is a child she doesn’t know the details. She hasn’t seen the Filtration process. Lockhart has with him the only survivor of the Baron’s bloodline who trusts him enough to kill her father to save him. Lockhart also knows that Hannah, as a fetus, survived in the aquifer. He understands that there is something special about Hannah. Putting all this together, Lockhart is riding away with a prized bride and the knowledge to eternal life. This puts a smile on that face! " (source : http://www.thisisbarry.com/single-post/2017/09/20/A-Cure-For-Wellness-2016-Movie-Plot-Ending-Explained)

More or less along the same lines (this is no happy end), this one goes even further, giving to the eels the role of the master of puppets : https://www.reddit.com/r/movies/comments/5uju5i/official_discussion_a_cure_for_wellness_spoilers/diz1kjj/