Yes, of COURSE he's the killer. Don't get hung up on what you consider to be inconsistencies the whole story is told from the perspective of his feverish madness.
Otherwise we are expected to believe:

1. that strange hooded persons swarmed a mostly empty school for absolutely no reason
2. that they did so at the PRECISE MOMENT that a teacher was becoming seriously unhinged and seemed to have finally completely cracked
3. that there are murderous people in the UK who have absolute voids for faces
4. when you repeatedly stab one of these void faces, it just lies there without squirming in pain or making a sound
5. the void faces completely savage everyone the come face to face with but for some reason leave Robert alone and unharmed
   They can't harm Robert because they ARE Robert.
   I'm glad the filmmaker left us to figure this out for ourselves once it dawned on me that he was the killer and we were seeing the story told through his deeply disturbed eyes, I expected that last scene when the security footage was reviewed and he was seen carrying out the crimes. I'm glad it wasn't tied up in a bow for us.
   Here's a suggestion. Go back and look at the very end after he's been driving a bit. Notice how, in those last few frames, suddenly the lighting and sound dramatically change and he looks himself in the rear view mirror. That is the moment of clarity for him. Watch that again and see if you disagree.
   The movie is even called "F" the event that sent Robert on the downward spiral that we get to watch for the rest of the film. Otherwise why even bother showing how he has deteriorated up to that moment? It'd be like splicing two different films together.

Dudes, Anderson is not the killer. At least not the actual, physical Anderson. Having watched the movie twice, I am sure of this.
There is no way that all of the characters would have interacted the way they did, so differently between Anderson and the hoodlums, if Anderson himself was the killer. There is nothing in the movie to suggest that Anderson himself did any of the killings.
There are interpretations that the hoodlums are extensions of Anderson. There is more evidence for that than for Anderson himself being the killer.

I watched it a few months ago on (i think) BBC Three

Good point Feriens did you also watch it last night?

I'm saying the exact opposite lol
There's a theory on this board that the ninjas were in Anderson's head and he was actually the killer. I'm saying that that surely cannot be the case because his daughter doesn't seem scared in his presence despite (going by the theory) him stabbing her!

You suggested Anderson might be the killer.
No he didn't! He is suggesting that he is not the killer.

Er, yep. You suggested Anderson might be the killer.
I was replying that IF he was the killer, why would he vomit at seeing the body of that woman, which- IF he was the murderer, would not be so reviled by.

Because he's just discovered a beaten body!

So why do you think he spews up when discovering the woman's beaten body?