I disagree. In those movies, we are emotionally invested in the kids who are trying to stay alive. We know they're not all going to make it, but we want someone to get out alive. More of us identify with the victims than with the killer. Sure, the killer might be more dramatic, more 'larger than life', maybe even more interesting -- but we CARE about the people who are fighting for their lives.
In every story, we need to care about somebody. I don't know about you, but I'm not going to care about a psycho who is mowing people down with a car for some vague and flimsy reason. (Even if his reason were solid, I still wouldn't care about him. I'd want him stopped.)
Yes, mostly we don't root for the maniac - though sometimes audiences take the attitude that they want him to 'succeed' at least so there's another sequel because he's so much fun to watch (Freddy Krueger?). But yes, we are scared on the behalf of the victims etc. - but that's not the same thing as being particularly invested in the victims... I mean, we feel the fear through them, they express our experiences and our fears of the maniac etc., but it kind of depends on the genre or film. We care about the person depicted by Mia Farrow in Rosemary's Baby, but do we really care about any particular kid in a slasher movie (beyond experiencing terror vicariously through them)? I think that depends on the movie. So, for the kind of film where that matters, where it matters that we become emotionally invested, you may draw a more complex character. But for others, it's just a warm body - you don't really need to spend much artistic effort on a fully rounded character whose sole function is to get slaughtered. I mean, my suggestion to the OP is to make the second kind of a movie, where OP doesn't even need to start a thread like this because there is nothing to the "hero" character. I'm saying, for his purposes it would be easier and just as effective artistically to make the second kind of a movie - all about the maniac and the "hero" or "heroes" are just warm bodies.