"So why not let him know what the plan was?" Spike isn't following the logic at all. For all his questionable honesty, from incomplete explanations to outright deceit, when he hatched a plan with somebody he let them in on all the details. Whether or not that plan survived contact with the enemy was another question entirely, but the basic plan was always shared. Deliberately withholding that kind of detail from one's partner is just beyond his ken.

Spike wants to just leave it at that but the serum means he can't. Fucking hell. Or Hell, as it were. "I wanted out of the life. I should have found another way. She could still be alive if it weren't for me but I was too fucking selfish."