The conceit of LHM is that it is a video diary of a serial killer recorded over a generic teen slasher movie. Max, the serial killer protagonist, is a dilettante affluent middle-class Londoner, with a career making wedding videos. He's a very believable character in many ways; self-assured, mildly arrogant. The writer and director have deliberately avoided many of the stereotypes of the serial killer, there are hints as to why he does it, but no easy answers. He has a comfortable existence, an active social life with friends and family, no sexual motive.
The gimmick is used effectively to challenge the audience as to why they watch horror movies in the first place. As such, it is something of a film made by a horror fan for fans. I also feel that the director is making quite a social point, most of the victims are also affluent types, being killed in their BMW 7-series or told that if they wanted neighbors to hear then they shouldn't own so much land. The underclass encountered tend to be spared, or recruited.
The video-diary format makes good use of a low budget. Lighting is harsh and camera work largely hand-held mini-DV. The lead actor is good, but I feel that some of the others were less good, and weakened the overall impact. Oddly I didn't find the horror aspect to be particularly effective overall, but other people seemed to "get it" more. I think I'd had my usual failure to suspend disbelief and be fully absorbed into the film.