While anime villains are capable of profoundly unsettling acts of treachery, not every antagonist is easily understood as good or evil. Some characters are constantly oscillating between sides, and following up horrible deeds with incredible acts of kindness. Others exist in a perpetual gray area, as they commit atrocities in the name of an unarguably noble cause. Morally ambiguous anime characters are complex, multi-dimensional figures who keep viewers guessing (and arguing) about their true intentions.
L is a world-class detective who's been solving seemingly impossible cases since he was eight. One of the keys to his success is his reliance on unethical deduction tactics — such as torture — to get suspects to divulge information. Perhaps the most extreme example of L's methods comes when he makes Light Yagami's father pretend like he's going to murder Light and Misa, just to see if they'd use Light's power over life and death to save themselves.
On the one hand, L gets results, and his work saves lives. On the other hand, he disregards human rights to achieve his goals. It's hard to know what to make of L, and that's part of what makes Death Note so interesting.
Elias Ainsworth of The Ancient Magus' Bride is a part human, part fae mage who often uses his powers to helps others. He's kind to Chise, and gives her a home where she can learn magic from him. However, Elias also has a dark side. He only becomes involved in Chise's life after he buys her at an auction.
On top of that, he often obscures his intentions and his violent past, which involves consuming human flesh. Can Chise trust him? Should she love him as much as he does? It's hard to know.
Levi Ackerman of Attack on Titan is willing to sacrifice his life to defend humanity from the man-eating Titans. Thanks to his tireless efforts, countless people are kept safe, and he displays great compassion for his subordinates, whose deaths leave him visibly upset. However, Levi isn't above using violence to get his way.
When Eren Jaeger is arrested and taken to court, Levi establishes dominance over the Titan shifter by beating him severely. When combined with his criminal past, it's hard to know how to read Levi.
Death Parade is a series about people who believe in objective morality. Decim is an arbiter, which means it's his job to decide which souls are reincarnated, and which are condemned to oblivion. He discerns this by watching how the recently deceased react to elaborate, sadistic games.
As the series progresses, Decim realizes he cannot accurately judge human behavior using crass and violent methods, which means he can no longer properly do his job. Watching Decim struggle with moral ambiguity both within himself and within the souls he judges is fascinating.