It's basically about a teenage serial killer who may or may not be a vampire, but even he comes out several times and says it doesn't matter, and breaks vampire rules (like going out in daylight.) It's sort of meta (in 1977, when things weren't typically meta.) Romero said he thought Martin was just a screwed up kid, and as far as I'm concerned the point of the movie is to paint life in a decaying Rust belt town in the middle of the economically destitute 1970s, and ask if in such a setting, whether or not it even matters whether or not this kid is a vampire. The movie doesn't spell that out, either.
The actor who plays Martin, John Amplas, turned up as a Puerto Rican gangster in Dawn Of The Dead, and a scientist in Day Of The Dead.