Open-ended interpretation. Also, odd things I noticed.

I think the hoodlums, like the children in Ils, were intentionally left open to interpretation. They could be spiritual beings, or living; extensions of Anderson, or specific people seen elsewhere in the film; meaningless characters, or representations of antisocial youth, bad things or bad times; etc. But they weren't intended to be any one of these things, and evidence to strongly support one case over each of the others is intentionally left scarce. To me, films that make it clear there is a definitive answer for "who dunnit" are often not as interesting in some aspects, because once we find out who it is, it removes much of the intrigue.

Just my thoughts on what the hoodlums were supposed to be; you may completely disagree, of course. Also, a few curious things I noticed:

1) The hoodlums physically harm everyone they meet EXCEPT Robert, though they had good chances to - e.g. when he was locked in the studio, or when they stabbed Kate. This suggests a special relation between Robert and the hoodlums; they're not just randomly killing everyone they meet. This might support the interpretation that they are an extension of Robert. Or they're trying to scare him by harming others, for some reason.

2) James (the younger security guard) is portrayed as a possible suspect early on: he reads fitness magazines, turns up in suspicious places, and oddly locks Robert in the studio despite Robert having an otherwise good chance at escaping without the hoodlums. We never actually see James killed or even harmed. When the hoodlums surround him and he starts acting distressed he almost seems to be joking around.

3) Jake is also portrayed as a possible suspect, e.g. in the bathroom stall when Kate says she wasn't expecting him for ages. We never seem him die and his mutilations and state of distress seemed faker than usual. But his apparent confusion and distress when he first encounters the hoodlums suggests otherwise.

These are just things that I noticed, and I think could be interpreted as suggestions that the hoodlums may be extensions of Anderson, or specific characters, etc. But again, I think, on the basis of a lack of evidence that strongly supports one theory over all of the others, it was intentionally left open to interpretation and the hoodlums were not meant to be definitively one thing.