Personally, the most evil thing Angel ever did was allow his show to be cancelled. Seriously though let's run down the list:
Letting Darla/Dru/Harmony live
I suppose this is the first of the "Inaction vs Action" argument. I suppose th argument becomes: is it worse to kill or let someone die? To a degree Buffy and the other Scoobies have known that Dru and Spike were around killing. Hey even Buffy wanted to send Vampire Willow back to her dimension.
I'd still contend that Angel deliberately killing someone relatively would count as a greater moral trangression than failing to kill a vampire. It may be that emotionally he was unable to do it because of the familial feelings he had for at least Dru and Darla. This is similiar to Buffy failing to kill Angelus. Her not killing him wasn't a moral decision. She didn't believe it was the right thing to do. We know Angel struggled emotionally to kill Darla unless, he saw her directly threaten somone he cared about or was at least innocent.
Refusing to help Buffy in pre-show/S1/S2, in spite of seeing her in the risk of death every day
Angel did help Buffy. He saved her and the Scoobies lives. Against more dangerous foes, it's possible that Angel wasn't sure of his own power. To his credit, he became more and more involved as time went on. I see this as him believing in himself more and more.
Setting a bunch of violent gladiator demons free
The Ring introduced us to the concept of demon rights. The concept of human rights is instinctual for us because we are human. Even criminals have rights in most civilised societies. Demons aren't not human but all of them seem to sentient and many sapient. Arguably their 'rights' would depend on how an individual demon treated humans. The demons that were set free at least seemed affable. Trepkos, for example, seemed quite honourable. In any case the McNaramas were the real villains of the piece as they clearly don't value demon or human life. The best thing is relying on the system of research and detroy (where applicable).
Helping Darla/Dru kill W&H lawyers
It has to be said, the lawyers did bring this upon themselves the main problem is that Angel's actions were based upon anger rather than justice. This temptation into darkness is exactly what W&H would want. Also, what about the lawyers friends and family? Did they deserve to die?
Performing dark magic to get Connor back
Yep, pretty ****ed up. I like that Gunn called Angel on this. The problem is that Angel tends to act rashly when he cannot deal with his emotions and that is bad for the people around him and the people he is trying to protect. No evil per se and it's a trait that needs to be kept in check.
Allowing his soul to be extracted
This wasn't even Angel's idea and he did it with the best of intentions. He also wanted to make sure the gang made provisions. If anything it was incredibly selfless. Angelus is the thing he fears people seeing the most. To me it was the actions of a guy willing to do whatever it takes to defeat the perceived Big Bad.
Global memory modification about Connor
I wrote this on another forum:
Well the moral issue for me comes to down Angel taking away something that he didn't have the right to take away. All our memories form a part of us and we use such information to make decisions.
However we cannot reasonable argue if any of the Fang Gang would have objected [to joining W&H] if it were not for the memory wipe. The reason being, we still don't know how much the Connor-related memories would have affected their decision.
Selling himself to W&H
I don't actually see this as evil so much as misguided. Incidently, it doesn't actually in itself negate the good that Angel did under W&H employ. It was in a way healthy that Angel felt uneasy about working for W&H. If we worry about our moral certainty, there is hope for our moral salvation.
Manipulating his friends on board for the suicide assassination mission
If by manipulate, you mean, be honest about the danger and intent, then well...This is actually insulting to Fang Gang to think they lack that much agency.
Allowing Twilight to control him
Yep, that's the one. Killing 100os of people because he couldn't be bothered to question destiny, which has been shown time and again to be mutable and up for intepretation. I don't know who I'm loathe to forgive more, Angel or the writers.