So because Spike loved him, he would watch him die.
That line so perfectly sums up Spike's thoughts - all his tortured agonising, leading to that one conclusion. I'd say that was pretty close to selfless love from a demon (at least, as close as he can get).
Very nice.
ETA - [deleted previous ps] Ignore me, I'm talking rubbish. I got mixed up with names - never my strongest point.