So Liz is missing presumed dead?
and Richard killed her?
Where is that inferred?
I don't think it was inferred that Dean killed her but his look strongly suggested there was something shady about it, that he knew exactly what happened and probably involved.
But there were other hints in "Darkplace" and "Man to Man with Dean Learner" that led one to believe Dean was responsible for "accidents" and death of some other people and from the way he talked about and treated cat, his mother and other things, he was probably a sociopath.