If a creature with trample assigns enough combat damage to kill all the creatures blocking it, its controller can choose to assign any "leftover" damage to the defending player. Once this damage has been assigned, nothing short of damage prevention/redirection will stop it. It doesn't matter if the creature's still in play or not, after combat damage goes on the stack.
From the sounds of it, it's just a case of the blocking creatures killing the trampler through combat. Unless they had enough first-strike damage to kill the creature (in which case the trampler wouldn't ever get to assign its damage), it doesn't matter. All combat damage resolves at once, so the trample damage to the player and the combat damage to the trampler are simultaneous.