Unsympathetic Protagonists = Good
Are people really so desperate to be spoon fed blank, anodyne, cookie-cutter heroes in their films? Would this film have been much better if we had a bunch of likeable, good-looking high school kids fighting off the aliens? We could even have the jock, the hot girl and the nerdy guy who pulls through in the end to save the day and any other cliche you can think of.
Don't get me wrong, the mugging scene at the beginning of the film hardly endears you to the characters. That was the writer's intention, hence why he included the knife in it. They're not likeable characters at this point in the film. They're a bunch of kids who think they're bad-ass and all display a mix of cockiness and cluelessness and they're certainly not meant to be admired at the start. It's what they do throughout the course of the film that redeems them. They do save Sam (and she of course saves Moses at one point) and at the end, Moses realises that it was his actions that brought the aliens to the block and he risks his life to save everyone else.
I know I'm probably going to get a load of replies saying I'm a thick as hell liberal but I really don't think of myself as one. I don't think we're supposed to instantly forgive the characters for the mugging just because they were as scared as she was, it was still a horrible thing to do. But the events of the film show that they can change if given the chance to and that they can realise the mistakes they made, as Moses explicitly acknowledges when in the Weed Room.
One of the best moments in the film (for me) was the one where Sam notices Moses's Spider-man duvet and asks if he has a little brother. It just reinforces that these guys are just kids. That in and of itself doesn't excuse their actions and it's not enough to say "I had bad role models like Hi-Hatz" as a defence to mugging someone but when someone is living by himself in a rough part of town with his only adult influence being his uncle occasionally dropping by, you can understand how he'd gravitate towards those people who are seen as cool and powerful in that microcosm of the block. In any case, I think it's much more interesting to watch a film where you start off disliking the main characters and then realise that they do in fact have the capacity to change, they've always had the capacity to change and do in fact realise this potential when faced with a greater danger that makes them realise they're not gangsters, they're just a bunch of kids who think they're tough.