I got ahold of the manga, and in the chapter where Light's saying he's going to kil all the criminals with a heart attack, he also says he'll make the useless people die of illness. Makes ya think he's a biiiit meaner that he's letting on.
When he is eventually found out, it'll be interesting to see what they'll do to him, as it'll be a bit...*shrug* odd to just...kill him for mass murder, that's that. Who knooows. Maybe they'll join him with his quest for a perfect world!