Mikami considered himself righteous, certainly, and not evil - the evil were the unrighteous, the ones that he had to eliminate, by one way or another, to better the world. Good or bad, though, that was something that he didn't really care about; the rest of it was all much more important.
He'd mostly agreed to Grell coming over to see if he had a Note - and to try to get one from him if he did. Mikami had no intention of killing anyone with chainsaws - it was impractical, for one thing, messy for another, and would require him to go out and chase people down, which was ... not the best way to judge people, in his opinion. "No shinigami that you know of use notebooks?"