Most people know Loki as the Norse god of mischief and/or evil. In old Pre-Christian mythology he's more a god of chaos--the Lord of Misrule. In later tales he begins to shift toward evil, taking on aspects of the Christian deity of evil.
In Marvel comics, particularly the stories of Thor, Loki is also considered a villain.
I, on the other hand, wanted to something different. In my universe Loki's one of the immortals, a race of beings who fled to an Earth very much like our own after their own world was destroyed. Loki's a scientist, though not what anyone would consider a particularly good one. While most scientists would prefer to make a rational, reasoned approach to things, Loki tends toward gut-level inspiration and experimentation. Which, as one can imagine, leads to a lot of trouble.
Take "Loki's Sin." It's the title of my very first book, as well as a term coined to describe what Loki did. he has a habit of going off half-cocked and doing things without considering the possible ramifications. The law of unintended consequences has Loki's name written all over it.
In trying to recreate the virus that made him immortal to save the life of a young woman with whom he'd grown rather attached, Loki accidentally turns her into a vampire. The first real vampire, as a matter of fact. But he's just getting started. He also discovered that humanity had lost its ability to work magic not through anything we had done, but because the alien Cen (or Centians, in the first book) had damaged our DNA to take away the gift. Loki introduced a series of nanoviruses to fix this malady, as well as other perceived maladies, but didn't really grasp what he was setting in motion.
Loki killed tens of thousands of people as their bodies fought and rejected the viruses. Those who survived, however, were enhanced in various ways. They became superheroes and wielders of magic.
He'd honestly only had the best of intentions. Unfortunately, we all know where that particular not-so-yellow brick road leads.
So Loki's neither hero nor villain in my universe. Just a guy prone to making really big mistakes.