"I want the Spike who tried to kill me when we first met"
He's a vampire. It's practically his job to do this, not to mention she literally asked for it. And in that way, he's doing everyone a favor. She may not admit it, but she still has the memories of how it was before, and knows how hard it is for her to remember and keep going with her head high. But she's beginning to slip, and it's only a matter of time before someone else sees it. He knows she wouldn't want that, wouldn't ever want anyone to see a sign of weakness from her.
But even so, hands pressed on either side, he holds the pillow down, smothering her, and he feels grateful for the small hand that wraps itself around his wrist, not fighting him, but rather a sign of gratitude. A sign so much more visible then her slow decisions.