Motive: Wes has been betrayed by Rebecca, and the only way he finds out is by building evidence against her behind her back. That sort of betrayal can run deep, especially since Wes is the one who kills Sam to protect Rebecca.
Evidence: First of all, Wes kills Sam, so we know he's capable. Also, we see him standing over Rebecca's empty chair shortly after she "escapes." Plus, think about how freaked out he is at the end! Could it be guilt from killing Rebecca? The only thing absolving him at this point is the fact that he says he believes Rebecca shortly before her death.
Conclusion: It's entirely possible Wes did it, but he's not a prime suspect.