JAngo had to die, because Boba had to hate the Jedi for some reason. As for why he missed Luke, its the Good guy bad guy concept. the good guys always win, always get everything perfect, and the bad guys couldn't hit a supersized snail at point blank range. Notice that the clone troopers were really good shots until they started working for the empire; same concept. All comes down to movie rules and dynamics...in short, all bad guys are cannon fodder.
Sorry to break it to ya fellows.....
and there is also the thing that Lucas probably wasn't expecting a character with only three lines and almost no screen time to become on of the most populer characters ever......more loved than even Luke, Han, Leia, and Old Obi I think.....
"Ke barjurir gar'ade, jagyc'ade kot'la a dalyc'ade kotla'shya."--
Train your sons to be strong, and your daughters to be stronger.