In Reply To #14
humans have things called emotions though. they can, and often will take over your reason when you witness something traumatizing, be it sadness, helplessness, rage, anger, hate or such. al being half vampire also contributes to that.
alucard could've lost himself in hate too, but unlike drac who was a bitter 300+ year old vampire with his last shred of humanity lost when lisa died, al was still a teenager (supposedly) at that time, and was half human.
with time his emotional wound might have healed a bit, and he realised the error of his ways, thus leaving drac and betraying him.