As for the question in the OP, I don't think the Scoobies acted too harshly. I'm a huge fan of Spike (I would argue his development and his complete story arc is the best on Buffy), but from the Scoobies' perspectives, he had kidnapped their best friend, chained her up, and threatened to unleash Drusilla on her unless she admitted that she reciprocated some sort of feeling of love. It was completely reasonable of the Scoobies to react the way they did.
This topic strayed way off its original path, and there's absolutely no way I could attempt to tackle everything that's been stated. After reading the entire thread, here's what else I have to contribute.
During season 5, Spike was a soulless creature, who was inherently evil. Even when he started to fall for Buffy, he still was an evil, soulless creature. That being said, he did show that he had the capacity to do good. Intervention is a great example of this. The way he resists Glory's torture, which was something that went 100% against every instinct he would have as a vampire, shows he CAN do some good (and the reasoning behind it is irrelevant. Just because he was doing it because he loved Buffy doesn't negate that it was still a good action).
Someone said that you can't have it both ways; that either Spike has to be accepted as a soulless creature who can only do evil, or he always had the capacity for good, so thus should be held even more accountable for every murder he committed. Here's my take on this. As I've said, Spike is inherently evil, but he IS capable of fighting those evil instincts. Presumably, he was always able to fight those tendencies, but that didn't seem important to him until he started to love Buffy.
To me, that shows that he's a better man, or vampire, than any other vamp we see. Whether its because he is uniquely able to fight the evil within himself, or because he seems to be the only Vampire who DOES choose to fight it, the fact of the matter is, something changes inside of him, and he starts to make the choice to fight.
Do I think this lets him off the hook for all of the evil he had done in the past? No. He will always have to live with the decisions he made. Certainly starting to change does not make all of that just go away. But something changed during season 5 that made him want to fight against evil (both the evil within himself, and evil in Sunnydale). To me, this paves the way for his redemption, and for his becoming a hero/champion