Actually Kira does it because of a god complex and the second he believed he was "In the clear" his murders became less and less researched to the point where he was likely murdering perfectly innocent people. He wasn't really doing anything with the any pretense of "good" behind it except in lip service.
Though ignoring that he is often killing people who are already paying for their crimes or where their crime had so many questions that they may not have actually done it, I've heard of people falsly accused of morbid crimes where everyone thought it was them before, meaning Kira was knowingly risking the murder of innocents. Better yet many of the crimes he murdered for were sometimes not even violent.
Kira was individually reversing all the progress the law had made until then especially for Japan (this wasn't... lets say a place where Capital Punishment was legal. This was Japan where it is illegal to even use a gun on a non-violent criminal)
There is no ambiguity there. His motivations are flawed, his method is flawed, the effect is negative... That is without going into the other things he did.
Though the fact that I don't believe in murdering criminals may help a bit in understanding why I think that.
You then learn that an "entity" known as Kira kills him afterwards, wouldn't you rejoice? It might not be politically correct(which is clearly shown in the manga) but people would definitely cheer him on!
Not me. I don't believe that revenge is a viable emotion that should be enacted. As well from what I understand a lot of people who "want revenge" want some form of nebulous revenge done by someone else not infront of their eyes. If they watch the death they want it to be clean.
As well the people who "cheered him on" weren't people with genuin loss or who knew people who suffered loss. They were mostly teens who were bullied, or who were bullies, who would like nothing better then the idea that those who made them suffer should probably die. The fact that I was hearing this from people who would later write the laws, rather then the crazy nuts, was what I disliked.
-Not to mention they were fully in support of EVERYTHING he did.