I agree that the scoobies' behaviour was aggrivating to say the least, but I think Joss also made it realistic.
I mean, Buffy had left without a word for months. Put yourself in their shoes: your best friend has been missing for months, could be dead, alive, a demon, evil, not evil, living it up, or bleeding to death in some alley for all you know. You've been worrying and trying to deal with the stress of keeping the town safe when she's gone AND trying to find her, and then she shows up and you still don't know why she left in the first place. Then out of the blue, when you FINALLY have her home and everythings back to normal, she's about to take off again. They're not going to try and see her point of view, or stop to think about what would hurt her so much that she would leave, they would just react. My friend is leaving again- my friend is leaving me- my friend hurting me- my friend is being selfish. It's amazing how much anger and hurt can cloud your judgement and make you say things that you might not under any other conditions. What they said might not have been morally right, but it was real.
Same goes with Angel coming back. XANDER of all people (who hates, has been jealous of in the past, doesn't trust, and would overall like to see Angel as a pile of dust at this time) sees his best friend kissing the vampire that made his friends hurt and lives a living hell for almost a year. The last he saw of Angel he was fighting Buffy and trying to destroy the world. He would feel hurt, betrayed, panicked, worried, shocked. He tells the others, and naturally their reaction is similar, they're all worried and scared out of their minds and probably hurt that one of their best friends has kept something so important from them. They're not going to care how or why he's back, or why Buffy's protecting him, they're not going to be 100% rational. Angel who tried to kill them is back- Buffy is hiding him- Buffy is kissing him- Buffy is betraying them- something MUST be wrong with Buffy.
Although I'd have loved the scoobies to always be behind our blonde slayer, it just wouldn't work. The show has always had an odd element of reality to it (even though it is technically a sci-fi show), and it wouldn't flow with the characters or general feel of the show if everyone was always happy and understanding. They were irrational because HUMANS are irrational. Maybe not always, but we can be- especially when we feel threatened or hurt.
(Although as a sidenote, when I saw "Dead Man's Party" I was hoping for some vamp nibbling too😉)