I'm sure numerous people have various ideas on that question, but mine is something a little simple: Moral is that, no matter how good the intention is/was, anybody could consume themselves in a great power of controlling death itself, and end either way (good or bad).
-My friend came up with one that goes: Any gift or power can be used for good or evil, abused or praised, it's not destiny that controls those actions, it's our decisions.
But that's just our say in the whole thing. I love death Note for being so open when it comes to these questions ^______^
When I finished Death Note, I actually put a lot of thought into this, and here's what I came up with:
~There is no set "justice" in the world, and the line between "good" and "evil" is blurry. Light wanted to eradicate the criminals in the world, but in doing so he became nothing more than an heartless murderer. This relates to the death penalty- do we really have the right to kill a criminal, even if it's for a good cause?
~Even by killing off as many criminals as you can, you still can't rid the world of all evil. The world is a dark, dark place, you see.
~Power corrupts. We all knew this already, and Light is just another example. He started out determined to save the world but somewhere along the line, his only goal became to rule over it as a "God." His power went to his head and he became evil and maniacal- he became "Kira."
~If you play God, you will be shot. Multiple times.
I guess there were other things, but I can't remember them at the moment.
I would say, that Death Note shows us how ambition can be corrupted by mankind having power that is far too great for them to possess, and that it is not one's place to pass judgement on whether someone is deserving of death.
So overall, an ambition fueled by a strong lust for power cannot succeed once one's morals and values are deteriorated. So long story short, moral of the story = justice always prevails.