Have any theories of why Snape killed Dumbledore?

snape is a good guy, everyone underestimates dumbeldores cleverness. he knew the death eaters were coming, he knew the horcux wasnt there. in the 6th book harry overheard snape telling dumbledore that "he didnt want to do it" and dumbledore saying that "you agreed to" which probably refers to the deal that snape made to kill dumbeldore when the death eaters arrived. right before snape killed dumbeldore, dumbeldore said "severus....please..." why owuld the most powerful wizard be pleading before his death. snape was looking at him with hate right before he killed him, because he hated what dumbeldore was making him do, why he did it, i dont know. snape is clever to, he made the unbreakable vow knowing dumbeldore wanted snape to kill him, very clever on snapes part