How does Cordy being killed translate to justice? For Cordy to be inhabited, used and then killed by her friend would be the most unfair thing to ever happen in the Buffyverse.
I think the "most unfair thing to ever happen in the Buffyverse" would have to go to demon hunter Holtz rushing home only to find his wife has been raped & murdered and his daughter turned into a soulless demon that he now has to kill while she pleads for her life. And then when he's transported centuries into the future for vengeance, the two vampires who reveled in destroying his life are alive, seemingly still together, and expecting a miracle baby even though no vampires in history have ever conceived.
Or that Drusilla is the only member of the Whirlwind who was never given the chance to redeem.
Cordelia chose not to tell anyone the visions were deteriorating her brain. She hid it for nearly the entire 2 years that led up from receiving them in
Parting Gifts to being in a coma in
Birthday. It's no different than Riley not wanting to give up being a super soldier and almost dying of heart failure.
Cordelia chose to believe if she never ran into Angel at that party in
City Of, by her 21st birthday she would be a famous wealthy celebrity who stars in her own self titled tv series and everyone else's lives would be in a world of misery & chaos without her and she'd have to save them all. She believed Angel could survive having the memories of raping, torturing, and murdering thousands of people over 145 year span and of being trapped in Hell for 100 years but would be huddled in a corner pissing himself without her being his friend the last two and a half years.
Cordelia chose to get infused with the DNA of an unspecified species in order to keep the visions because she wanted to be needed. It wasn't just about helping those who need it, it was feeling superior, a Champion, integral for other's survival. "You can't fire me, I'm Vision Girl." She moved to Hollywood to use her looks to get wealthy and adored because that's what made her feel validated but then learned that everyone in Hollywood is as pretty as she is and most of them actually have talent. AI paid her bills and gave her validation. She became as desperate as Willow with the magics. It wasn't selfless. Unless someone goes by the Anya as a vengeance demon definition.
Cordelia chose to believe that because she suffered for 2 years she was the best of mankind and that the universe finally noticed she was a Higher Being and left Earth for an unspecified dimension to do unspecified work. Every step of the way she made the wrong choice and was called extraordinary and praised for it.