it's normal to feel like this. this too shall pass.
if there was any one situation that made me immediately conclude that your father is an asshole it was this. i'm absolutely flabbergasted. i'm canadian, which means i'm generally pacificist, but i feel inclined towards violent activity towards him. argh.
what can you do? yes, you could have kept him. and maybe he'd still be alive. but then again maybe he'd get wiped out at 4 months instead of at 6 years. life is crazy, fucked up and weird. you can't assume that your actions caused his death. you did not kill him directly or indirectly. self-recrimination is normal and healthy, but it's healthy to recognize what's realistic versus what's not. you made the decision that you had to at the time.
you are *not* a horrible person. and this is *not* your fault.
was your decision to get rid of him based on your concern for him? how can you fault yourself? you did what you had to do.
treasure the joy that you brought into the world, the good memories that he left everyone with. use your support system, there are people out here who care about you. when it's right for you, grief can be shared.