I really liked Death note, until
after L died: it's should have ended there with light winning.
If I had it I don't think it'd ever use it, but it would be worth keeping in case of emergences, I mean what if you had had it during WWII? I mean we don't know that killing Hitler with 'natural causes' would have ended the war, but it would at the least weaken things surely, possibly leading to a quicker end to the war, and less dead all round? Who knows.
Overall: 'Is it wrong'? answer: Yes!
However 'Is it wrong to stand aside and let others die?' Also yes!
Your working with morally grey; it's no longer a matter of good/evil. Things are rarely so simple; There are situations where its justifiable to use it. Not, as light often did it: as a justice things, not punishment, or revenge: if someone committed murder and is now in jail they wont be doing any more harm, so there's no need to use it, but say a serial murderer was still on the lose, still killing: then you could save lives. Of course then you have to worry about if you've got the right guy.
My point being its not something you could say is or isn't the right thing to do, it depends on the exact situation at the exact time, you couldn't say it could never be the right option, or that it would always be the right option, but then of course whose opinion as to whether you use it in a given situation should you go with: we all think our own view is the right one (otherwise it wouldn't be our view), but no one can really know the consequences of their actions before hand, predict all you want, you'd have to be omnipotent to really know. And of course the more you were to use it, the easier it becomes to use it again and again, causing people to question why all the bad people in the world seem to be dyeing conveniently (see plot of death note XD).