in short, conflicting but complimentary unbreakable vows.

the theory: when snape wanted back in with the good guys during voldemort's first reign, he did so after harry's parents had been killed. dumbledore suspected that voldemort would be back and that harry, because of the prophesy, would have to deal with him.

so, dumbledore made snape make an unbreakable vow before he came back in. the vow basically said that he would betray voldemort only if harry was killed or if snape's betrayal could get harry his ultimate shot with voldemort. part of the vow would have been that snape continue to work in voldemort's service and pass him secrets so that voldy would continue to trust him and do nothing that would alienate that trust.

so, when severus is confronted by bella, he has to make that unbreakable vow. it simply cannot get back to voldy that snape is refusing unbreakable vows that make him support voldy's biggest plans, because voldy would cease to trust. dumbledore and snape have probably planned for this eventuality, with dumbledore reminding him of his vow and that to not kill dumbeldore would betray voldy's trust and break snape's vow. he would have also impressed on snape that harry is much more important than dumbledore. snape would have been horrified by this, of course, but his sense of duty and commitment to the cause and trust in dumbledore would have led him to follow orders however reluctantly.

good theory?