One thing that really interests me about Death Note is that it never makes any effort to humanise the criminals Light kills.
Light is very much portrayed as a villainous character. If you asked me what I'd expect from a series in which the protagonist kills murderers and is depicted to be in the wrong for this, I'd probably say that I expect two main things: I'd expect to get to know the human side of some of his criminal victims, and I'd expect it to be brought up that some of the people he killed were wrongly accused.
Death Note never does either of these things. The victims we get to know are the ones Light kills for being in his way, which makes sense, as those are the ones who come into direct contact with Light. The people Light kills for being criminals are only a name and a face to us, just as they are to Light himself. But killing them is still depicted as a villainous act.
The reason it's bad to kill criminals isn't that people are more than their crimes (although they are), and it isn't that you'll accidentally kill innocents in the process (although you will). The reason it's bad to kill criminals is that it's bad to kill people, and Death Note doesn't feel any need to make arguments beyond that.
It's interesting to think about! Because I do believe that people have a right to life, no matter what they've done. But, if I were writing Death Note, I don't know if I'd be brave enough to stand by that statement alone. I'd want to highlight redeeming features; I'd want the reader to empathise with Light's criminal victims.
More insight into the criminals Light kills might have been a nice addition, but I sort of have to respect the fact that Death Note just goes, 'Light shouldn't be killing these criminals, and whether they have redeeming features is completely irrelevant.' It's wrong for Light to kill these people, because they are people. You don't need more of a reason than that.