Integra thinks he speaks in an American accent just to piss her off and, to some extent, she’s right. She knows that he can speak a dozen languages and imitate any dialect he pleases, so if he’s speaking in an American accent is because he wants to, and he knows it gets on her tits because she’s English and, really, most American things will get on her tits.
What she doesn’t realise, however, is that his accent is really something of a tradition. She thinks he might have done it to annoy her ancestors, which is also almost true, but she never really considered what an American accent meant to the very first Hellsings; back when there was only one L in their name and Alucard’s name was written backwards.
Alucard likes Integra – in fact, he’d go as far as to say that she’s his favourite Hellsing so far – but that doesn’t mean that he liked all of his keepers. In fact, he rather despised his first: the bastard who locked him up in spells and restrictions and made him a pet. He despised him enough to remind him every day of the fact that the Hellsing Organisation had not won every battle against the forces of the night; that one or two of its earliest members had given their lives to the enemy.
An American accent wound Integra up a treat, but to hear the voice of poor lost Quincey Morris out of the lips of the monster that killed him… well, it had broken Hellsing Number One into pieces.