Light, L, Near, Misa, and Mello are all pretty dubious characters. I wouldn't say that about everyone though - Saya seems like a solidly good person, Matsuda seems like a solidly good person, Light's father seems like a solidly good person, Ray Penber and his wife both appear to be good people.
As for Light specifically, he's a mass murderer operating a reign of terror. Yeah, crime is down, but it comes at the cost of everyone living under the threat of death, and with people being killed for increasingly petty "crimes", up to and including seeming lazy. Even if only the results are looked at and Light's ridiculous god-complex is completely ignored, things look bad.