Mostly, I thought, "You stupid, ungrateful little twit!" and some other expletives not appropriate to be posted here. I fully realize the severity of what Spike had done, but if Buffy wanted to exact any sort of revenge on him in retaliation, that was her right, and nothing that Dawn should be either obligated or allowed to butt into. And before anyone here starts belaboring me for such an attitude, let me explain.
First of all, this was an issue entirely between Spike and Buffy, and Buffy was perfectly capable of extracting any sort and degree of revenge or restitution that she felt was appropriate. It didn't involve Dawn, Xander, Willow, Giles, or the guy with the cheese tray. This is not a case of a man overpowering some helpless woman. Both Buffy and Spike are super powered beings, as nearly equal in strength and skill as any two characters in the Buffyverse, which of itself is proof that Dawn, whatever her motivation might be, would be totally unable to carry out any such threat on a being who could hear her approaching from a city block away and smell her from the other side of town. Just about the only chance that she would have had against Spike would be to borrow Buffy's rocket launcher and take him out from long distance. And on top of everything else, she had warned him in advance. At best, he would be prepared for anything that she might try, and at the worst, he could have found her alone somewhere, and even with the chip firing, killed her so quickly that she wouldn't have any chance whatsoever to defend herself. That takes care of the stupid part.
Secondly, no matter what had happened between Buffy and Spike, it in no way negated the fact that he had been her friend, supporter, confidante, and defender practically from the first minute that she popped into existence in Sunnydale as a shallow, self centered, spoiled little brat. He had suffered for her and literally saved her life on more than one occasion, and had never hesitated a moment in doing so. All of that loyalty, pain, risk, and comfort should have earned him at the very least the opportunity to offer some sort of explanation for what he had done and what he was thinking at the time. But in actuality, that last part goes back to the part where what happened was entirely between Buffy and Spike, and none of Dawn's business, sister or not. That's the ungrateful part.
Now, I know that there are more than a few people here who are going to disagree with just about every word of this post and accuse me of blind support of one of my favorite characters in the Buffyverse, but that's the way that I see what happened. And as Buffy herself said later in the season, in the time and place where Dawn, Buffy, and Spike lived, there was no place for vendettas. The mission had to come first. With her new found maturity and insight, Dawn should have seen that quite clearly.