Good and evil are relative to begin with, the very concepts of good and bad are at the very least complex and seem to vary according to circumstance, or at the very least when society feels like it.
So in order to make things simple I’m going to suggest that evil is anything that poses an existential threat to society, by that definition I would also like to suggest that bad can be used to describe anything that hinders societal progress. That said, we can easily infer good is anything that contributes to societal process.
Now note by these definitions I would also like to suggest that just because something is not good does not necessarily mean it is evil! Water can be hot or cold but it can also be warm, a state which is neither.
So in the case of Light, is he good or evil. Light has done some pretty horrific stuff. He has killed, deceived and manipulated others all in order to reach his goals, but I find it difficult to plainly tag his actions as good or evil!
Killing by no means is pretty but Soldiers do it all the time, does that make them evil? The act in itself cannot be said to be evil considering we kill half the things we eat, and then we come to the matter of taking a human life. Typically killing a human being is detrimental to society but in the case of killing a mass murderer is it more useful or detrimental to society in that particular scenario?
Kira obviously believes the ends justifies the means, but the unfortunate thing about taking a human life is there really is no way of placing value on it. A mass murder now could be something completely different tomorrow.
In that instance would it not be more prudent for society and therefore more beneficial to reform such mass murderers. One could make the case, are they worth the effort and risk?
Is Kira evil? Not even deathnote could conclusively state so because in truth he actually wasn’t but he most certainly was not good either. He was a man, who deluded himself into thinking he could be a god. A mere man with mere ambitions who followed them to the bitter end, and after all is said and done…can we not say the same of ourselves?