Did he really "help"...?

After viewing this movie (which I actually enjoyed while watching it), I was left with a lot of questions after. One, in particular, was about the scene with his mom.

Did he really "help" her in a mercy-killing, or did he just outright kill her?

Yes, we see her first slash her throat with the glass, but what we're seeing was from Jerry's P.O.V./flashback, something we know to be faulty. As he kills both Fiona AND Lisa, he mentions them "suffering" and wanting to stop that for them (which is, of course, by "putting them out of their misery"). Sure, we could assume that it's him remembering that's how his mother felt (and odd way of justifying what he just committed) but how do we know that wasn't what he believed to be doing when he killed his mother? Could he have witnessed her "suffering" in terms of being mentally ill, and figured he'd end her "suffering" by killing her, without her making the first move?

It's just hard for me to believe that everything we see in that scene is true, when we know that his perceptions of everything are not accurate, and that point we know he has also been ill.

I'll also admit that I didn't get a close look at the newspaper article about him, which may explain it more. (Though I have a lot of questions about that scene too...)