if you think about it alucard's whole problem is that he's never really been very self determined. When he was human, he fought for God and Christianity. He felt letdown by God so he renounces and becomes a vampire that consumes others, body AND soul. Then he gets whipped by hellsing and becomes their attack dog. He was always leaving his reasons for fighting up to other people.
the major fought because he <3 war. he didn't care about germany, or the nazis ideals, he just got major war wood. He sees war the most brilliant display of humanity, in all its beauty and horror. So when he saw some fucked up monster walking around tainting the proceedings, he had to kill him.
He was truly a great _man_ (assuming he's dead)