Spike could anticipate what was coming the moment she threw her flowers towards him. That's the trouble of going away for a while: everything builds, and he can't say he doesn't deserve what follows. He has no corrections for her as she spells out his faults with the accuracy of someone who has known him longer than she has. The real truth, is that he's never been able to save anyone. -- She's seen it herself.

He doesn't move for the first half of her rant. (He's felt more like a zombie than a person these past few days, he may as well act like one.) Only when she attempts to stand, he takes the steps forward necessary to be able to catch her if she falters, even if it gets him slapped in the process.

"You're right, I don't have an excuse." Or he refuses to use it. Like he'll be able to keep that a secret forever. But death doesn't cover him when he's been awake for days now and he didn't come. Not to mention he died long after he lost her on the ship. "I should have been there."