Confused - spoilers

I thought the acting, especially Thomas, was sound ... but the screenplay left me scratching my head.

Was Will real or imaginary? I assumed he was a figment of Paul's fevered mind but if so, then why did that lady "witness" yell after him at the school at the end?

Did Paul kill the landlord and his mother? We see his sleepwalking, and there's more than one instance where we see him handling or near what could've been the murder weapon for the first killing. Then we see a flashback of him committing the second murder and watch him commit a third murder (if its real) of someone who may or may not really exist.

And the flashback to his childhood attempt to murder his father seemed to come out left field. That seems like a pretty drastic action by an adolescent and yet no one mentions it or, presumably, thought to get the kid some treatment?!

I can appreciate subtlety and ambiguity, especially in a psychological thriller, but this was just too muddled for me. I'd like to hear others' theories ...