Makes the most sense. It was a hostage situation. Alucard doesn't ever intentionally kill innocents, although often they get hurt along the way.
Given the choice, he'd rather give some one a chance.
And yet he has actually addressed this issue. Alucard said she was the type not to give up on life, which he finds extremely admirable. He thinks being a vampire is a horrible experience only some one with the right kind of will would be able to endure. He saw that she could do it.