The manga you are reading is so bad. I wonder what's the name of it and why you are posting some reactions to that manga in this forum which is about 4KotA chapters!!
What kind of redemption dude? Lancelot is literally resisting the huge push Cath imposed on him. Zeldoris clearly saw it and acknowledged it. Lancelot himself told Zel that his resistance has reached its limit. Of course it's horrible, but it's not Lancelot's own will. From the start, he asked his friends to stop him. He begged Zel to kill him. He asked Zel to run away. He tried so hard not to harm the DR residents.
Lancelot will change after this for sure. Like how Mael never become the person he was. But it doesn't mean they are completely responsible for what they did when their own will was suppressed by an external factor, Cath in this case.
Lancelot is the only one to treat Arthur and Merlin as criminals, and he is now experiencing an even worse nightmare. That's a great lesson here from the manga. A hasty judgment is easy, but it's not necessarily correct. And you have fallen victim to it yourself.
Lancelot is not OK with letting his loved one be killed, but others have proven to powerless to even help him. When he is saved from this madness, I hope he goes to Arthur and Merlin apologizing for his naivety and stupidity.
For every disaster happening after the awakening of Chaos, Cath is clearly the culprit, and his manipulations and schemes are evident in the manga. Arthur, Merlin, and Lancelot are other victims of his malice. They are not innocents, that's for sure. But there is no reason to develop a personal grudge against them when they were tricked, manipulated, and controlled by Cath.
Like
@Capalot98 said, 4KotA fans refuse to read 4KotA and their reactions are completely irrelevant!!
Right now, the one who has tried to oppose Chaos more than anyone else in the story universe is Lancelot himself. It's not his fault that other can't help him at all. His only flaw is his inability to let Guin die in front of him. And that's identical to what Mel, Ban, and Zel went through. They can understand him because they know the pain. Lancelot should take responsibility for his own decision, but not for the crimes directly committed by Cath. His redemption can happen, and he is still one of the KotA, the only people in NnT/MnYK world who can stop Cath.
So read the actual manga before writing an irrelevant analysis that ignores the most obvious information existing within manga pages!