Okay, so it's Walter who first asks Alucard in the manga why he decided to turn Seras into a vampire... and he uses a bullshit answer about Seras choosing for herself.
I'm still convinced that Alucard more or less manipulated Seras into becoming a vampire by presenting her with becoming said vampire or death, when he had plenty of powers and abilities to NOT put her in that position to begin with. It's human instinct to want to survive: of course Seras would "want" to take the other option if it means not dying. Thus, I can't buy the "it was her choice" excuse.