This is a difficult question. I believe that Willow is vastly superior person to Warren, but she tried to end the freaking world. She tried to end the lives of billions of people, billions of children, because she couldn't deal with her grief. She knew that she couldn't handle the levels of magic she was playing with, and didn't care one bit. She chose to access them anyways. That's not okay.
Willow is not the only person to lose someone. Buffy literally put Angel to the sword in S2. Spike, without a soul, had to deal with losing Buffy in S5. Wesley had the walls of his life come crashing down when Fred died. Angel had to walk away from Buffy, lose Darla to evil again and watch Cordy die after being inhabited by a world-conquering monster. Xander lost his best friend and still had the courage to help Buffy, as a 15 year old kid. Almost every hero in the Buffyverse has endured catastrophic loss, but only Willow tried to end the world.
Did she feel bad for it afterwards? Yes, she did, but her arrogance and her insecurity over her own self worth led her back to magic once again, even though she'd proven, pretty conclusively, that she had neither the will or the right to handle it. She butchered a man, no matter how bad he was that's wrong, tried to kill everyone she's ever cared about and tried to end the world. I don't care that she was "feeling the world's pain". That means nothing. It just makes her a terrorist. The world has been filled with arrogant people thinking they should kill to mold the planet to their whims. Willow as nothing different. The fact that she would even want to do magic again absolutely baffles me. It shocks me. What kind of a person would be so recklessly arrogant to start using magic again after that? And it's not just Willow. Her friends in S7 bear part of the responsibility for not walking the hell out on her when she starts up again.
Warren was a prick. He was a piece of crap who never comprehended what accountability means and who murdered people. He should have rotted in prison for the rest of his life. But if we're strictly comparing, Warren was a pathetic little man who killed because of his own self-weakness. Willow tried to kill billions, and then felt so bad about it that she started using magic again half a season later.
Who is worse? They both suck.
I think the problem with this assertion depends on the season. In S6, magic is a drug, Willow is the equivelant of an alcoholic trying to quit, cold turkey, failing and then going on a bender (and killing people in the process). If that still held, as a metaphor, I agree that Willow should never have done magic again ever.
But then in S7 Giles and The Coven tell Willow that the magic is
part of her now. That its not as simple as just giving up and not doing it again. (as The First, y'know, the untrustworthy evil thing, tries to convince Willow of this) That she can't afford to quit because 'something' bad is coming.
That's why Giles urges a deeply troubled and unsure Willow to go back to Sunnydale because she will be
needed, not because Willow goes, 'hey, I've had a few months of not killing somebody- back on the magics!'
Willow spends a lot of S7 pretty much crippled with self-doubt (with little support from her friends), but in 'Orpheus' we see a turning point, Willow learning to handle her power a bit better.
Would I have liked to see a Willow redemption arc as she struggled with this inner darkness? Would I have liked to see more consequences as she struggled with having blood on her hands? Would I have liked to see the Scoobies rally round her for more than about an episode before they all just forgot about it? Absolutely, but instead we had the Spike repemption show and Willow was relegated to 'romance with girl she has no chemistry with'.
🙁
Warren knew what he was doing the whole time, he knew he was accountable when he killed Katrina, the first thing he does is try and dispose of the body because he knows he's just committed a serious crime.
His solution? Frame Buffy. He's a calculating, odious little toad, but still dangerous.