Erik was a murderer, but he was like that because the world made him so. So, I guess you can say he wasn't really bad or good, just in-between.
In my opinion, the phantom's childhood had a very detromental effect on his life as an adult. i.e. the face disfigurement, rejection by his mother, being an orphan and growing up under the opera house. Erik is a wonderful genius and if he could have used his talents for good it would have been truely wonderful.
As said both in the novel and the musical Erik is a 'pitiful creature'. Despised by the world because of his genius and distorted face he retreats into darkness- i think the poor guy deserves to spit a bit of venom back at the world that has treated him so badly. we all feel sympathy for the 'Beauty and the Beast' thpy story, especially when it ends badly for the beast.