Snape Was Always Meant to Kill Dumbledore

The second reason that Snape agreed to enter the vow was that he was always going to

be the one to kill Dumbledore anyway. Though the task was assigned to Draco,

Voldemort never actually expected Draco to be able to complete the task.

This was his way of punishing Lucius for his failures. This meant that the task would inevitably

always fall on Snape to complete. Voldemort knew that Draco would fail. In addition to this,

in order to appease Dumbledore and protect Draco's soul, Snape agreed to kill the Hogwarts

headmaster himself. This would simultaneously complete the unbreakable vow that he had entered,

as well as protect young Draco's soul.