SPOILER - I always believed he...

I think the strong chance that he did do it was presented deliberately throughout the film, so I didn't feel like it was a twist that I uncovered too early. And of course, it turned out that he didn't exactly do it of his own volition. Throughout watching this, I was mostly wondering how the house, the canal, the ghosts and the current / past murders were connected. And we got some answers to that, at least.

I got to watch this with a director q and a, he said you were meant to know from the start and watch how the protagonist gradually realises the inevitable. When he has the visions on the public toilet floor at the start, you actually see him commit the murder if you pay close enough atention.

"To err is human...so...errrr..." - Gary King

"It's always the husband - every single time."

His murdering them is actually too convient. But not too much reason why. Him killing them defeats the ghost plot..maybe

That's because we know that from the start. That's just how the movie presents itself to the viewer and it goes from there.