Severus Snape overheard the first part of the prophecy involving the two boys who could potentially be the downfall of the darklord. It is possible that Severus Snape hated Neville because he knew that Neville Longbottom was the second boy in the prophecy. Therefore, if Voldemort had killed Neville instead of Harry Potter, Lily Evans, the woman that Severus Snape loved, might still be alive.