who is he?

Looks like a blast in a wall...

I am under the impression the movie trailer combined visualizzazioni of both parts of DH. There were shots of scenes with Severus that were not in Part 1 - so I think there was a combination. Part 2 is already completed, so they could have used some shots of it for the trailer. Maybe the pic te postato belongs to Part 2 if te had not seen it in Part 1.

Maybe it's Grindelwald being killed?

Sorry I can't help...

But the trailers (all of them) mostra scenes from both movies, as there are shots of V and Harry together, and they don't come face to face until toward the end, as well as other scenes not shown in part 1.