Death Note was attempting to show that "good" can result in very terrible, totalitarian things (from what I understand) by giving a genius with very high morals...the power to kill through writing a name in the titular "Death Note(book)."
Thing is, if I had a Death Note, I'd either try to collect all of them (and burn them) or just burn mine. I can't say I can respect the main character.

Code Geass, on the other hand, has the protagonist gifted the ability to, on eye contact, command any person (against their will)...and it is given to a person fighting Social Darwinism and Imperialism. Yes, they make mistakes, but they don't lose sight of their goals, ascend to the top, and dismantle the caste/class system before staging their own death in order to remove the position of "emperor" permanently.

I guess one thing that makes a difference is how solitary each of them are. The protagonist in Death Note is more of a loner, despite caring about the good of humanity. He is really just given a power, and uses it. He doesn't, I feel, grow or get better. If anything, the power isolates him further and promotes his hubris.

Meanwhile, in Code Geass, a large part of the protagonist is his loyalty to friends, and their returned loyalty to him. The source of the power he is given, a person who considers herself cursed with immortality, he makes smile and happy. Her, his sister, and his friends keep him...human. It is even stated that the power, the Geass, is supposed to isolate one from those around them, and yet he did the opposite with it.

I guess this is a good comparison of isolation, and corruption through power, and how it is not the power itself that isolates and dehumanizes people as the gain it, but how people utilize that power.