Me personally, I don't think Lucy from Elfen Lied deserved redemption. And here's why. Warning, this will be long.
The Diclonious is an allegory for the hardcore psychopath, and the show manipulates your emotions to give you a misguided sense of sympathy, to make them seem misunderstood (they are not). It is essentially psychopath propaganda and I suspect the author is on the spectrum.
Psychopaths have a deranged sense of entitlement. Shove them once, and they will think it's justice to gouge out your eyes (3 kids bully Lucy, all of humanity deserves to die).
If you have something they don't, it's unfair and they have the right to take it. Lucy murders countless families because she wants the comfort of a warm shower, but the fanboys will state a shower is necessary for "survival".
Psychopaths have social needs, but betray them and 50 years of loyalty can go out the window instantly (Kohta lied about one tiny thing after 3 days of kindness, she thinks it's fair to murder his family as punishment).
Psychopaths are narcisistic. Lucy views humans as insects, and sees herself as a hero in her story, and humans as evil. But this lie falls apart when she shows enjoyment in torturing Nana to death (same race as her) just because Nana insulted her ego.
Psychopaths have no shame, and only show regret when they get caught. She only apologizes to Kohta ONLY because it drove him away. She thought that by killing his family she could have him all to herself. But she didn't understand that normal humans don't "get over" death instantly like she does. Her apology is insincere since she thinks of his family as belongings she stole, but she doesn't regret the murders themselves. She has killed countless families after that event and swears to murder whatever family Kurama has left. Does that sound sincere to you? Humans are only pawns to her.
In the bonus OVA episode, Nana is about to kill Lucy while she's passed out. But to manipulate the audience into wanting her to live -- we see a flashback of her crying about the death of one girl (when she has already murdered other girls in the hundreds). Psychopaths can turn tears on like a sprinkler when it benefits them. Quick note on Nana, she seems to be a reverse-psychopath, a rare case of Diclonius that can actually feel empathy (she's too pure to kill Lucy even though it's the right thing to do). Thematically, it would have been a superior story if Nana was the main character, give her a kind of Hellboy dynamic.
I could go on forever. The point is, while things like Batman teaches you to understand psychopaths to help you escape their abuse, Elfen Lied teaches you to let your guard down and become a Harley Quinn. Harley's and Joker's "romance" is a quationary tale, while Lucy and and Kohta is manipulating the audience into becoming the victim.
The truth is, psychopaths are like wild animals.(And I feel ashamed to compare them to wild animals since I love wild animals) You can tame a tiger or a bear, but leave it hungry for a while and it WILL kill you, even if you raised it.
The Batman TAS episode "Mad Love" does the opposite of what Elfen Lied does. Both are stories about a character experiencing Stockholm Syndrome for a psychopath, but Elfen Lied says "trust them" while Batman says "don't fall for it". What's really the difference between Lucy and Joker here:
"You know... my father used to beat me up pretty badly. Every time I got out of line, BAM! Or sometimes I'd just be just sitting there, doing nothing. POW! Pops tended to favor the grape, you see. There was only one time I ever saw Dad really happy. He took me to the circus when I was seven. Oh, I still remember the clowns running around, dropping their pants. My old man laughed so hard, I thought he'd bust a gut! So the very next night, I ran up to meet him with his best Sunday pants around my ankles. "Hi, Dad! Look at me!" ZOOP! I took a big pratfall, and tore the crotch clean out of his pants!........ (Harley laughs) And then he broke my nose. (Harley is shocked) But hey, that's the downside of comedy. You're always taking shots from folks who just don't get the joke. Like my dad... Or Batman." Harley's response?
"It soon became clear to me the Joker, so often described as a raving, homicidal madman... was actually a tortured soul crying out for love and acceptance. A lost, injured child trying to make the world laugh at his antics. And there, as always, was the self-righteous Batman. Determined to make life miserable for my angel. Yes, I admit it. As unprofessional as it sounds, I had fallen in love with my patient. Pretty crazy, huh?"
But Elfen Lied takes that lesson, and reverses it, portraying it as a good thing. But the truth is, real life criminals and mass murderers tell stories similar to the orphanage incident and joker's stories to manipulate cops, parole officers, judges, juries, and criminal psychiatrists into releasing them... only for them to continue to murder and steal. Lucy is not a misunderstood soul and the Diclonious are not tragic outcasts. Lucy is a self aware and self rightious monster, and the Diclonious are abominations who's only instinct is to destroy everyone and everything they touch.
I rest my case. Sorry this was so long, but I couldn't find a way to summarize it. But enough of me. Who do you think did not deserve redemption?