Firstly I think Angel was absolutely looking for reassurance before telling Buffy the worst thing he ever did. But why, you cry? Why if he'd done all these awesome things in the past like fight demons and dragons and invade Nazi subs and hang with the Rat-Pack and Elvis, why was he so insecure? Oh, was is he just being manipulative of a 16 yr old girl?
Well...maybe its the fact that he'd never been in love before, never, not even as Liam. Not to mention that since his curse he'd very rarely been close to any human; and when he had it had always ended badly; y'know they set a mob on him and have him lynched or he killed them or turn them to save lives or something else bad.
So first, fragile love, coupled with all his past experience interacting with humanity plus absolute magnified shame about his past horrors (actually walking around telling him how he hurt them) is the magic recipe for insecurity.
And this is how it manifested; one question, one sign that maybe Buffy wouldn't look at him with disgust, pull out a stake and tell him to get the hell out. Insecurity about what you are, after what you've done and what you've been told over and over (that you're a monster) I think that's a rather human frailty to have. Certainly an understandable one.
Btw-this is why I think his own show enriches- for the most part- Angel's role on BtVS.
Also I don't ever think Angel would've thrown any love confession back in Buffy's face; 'Aha! No takebacks, you said it! You said it!' is not a S2 Angel thing to do, it would make light not only of a crime he feels condemns him as a monster but it would demean his own feelings for Buffy.
I think it's a fundamental misunderstanding of his character to in any way think he was capable of that.