Also you seem to be suggesting that Angel's thinking should've gone something like this 'Hmn, well Buffy''s beautiful and everything and that could be a problem. I better tell her I'm a vampire in case she falls in love with me and we get into a relationship.'
No, he wasn't thinking that - Angel was someone on the path to redemption, he was good and wanted to do good - but on the other hand, he didn't introduce himself as a vampire, a big part of what he was. It's a matter of perception (I don't know if I'm using the right word here); as people said on other thread: when Buffy was sleeping with Spike, Xander saw Spike naked in bed, doing something suspicious, with a invisible Buffy missing. He didn't think of the
possibility that Buffy was there with Spike – because sleeping with a vampire was something
unthinkable. Buffy knowing Angel was a vampire, would have created an immediate psychological barrier between the two. Not that she couldn't fall in love with him, eventually, but she would be more guarded.
I don't think, knowing or unknowingly, he was there to seduce Buffy-
He
was not there to seduce Buffy. This things, even in real life, aren't math – unless a person is a jerk like Parker (and I wonder why they did the nice-guy-who turns-evil-the-next-morning again) and does certain moves to get to a certain point.
So I get that his first reaction isn't to tell Buffy the vampire slayer that he's a vampire- he doesn't know her, maybe she kills him before he can explain the complexities of the curse.
Well, this would have happened if he had jumped on her on a dark alley and said "I'm a vampire but I have a s-" There were other ways to do it.
The thing is that Joss wanted use Angel and Buffy as a
metaphor, so he wrote it accordingly, IMO. It doesn't mean that Angel didn't love Buffy -
he was not the bad boyfriend of the metaphor.
Angel is clueless when it comes to 'Buffy as high school girl' and what she may or may not like in a guy/boy (see the 'why is Angel so awkward/clueless with women' thread)
Well, he is clueless, all right, but subconsciously he did things that were very appealing to a young woman - or an old one, for that matter.