I know you all are thinking one thought right now:
However, I come bearing good news. No longer do you have to wonder why Snape did it. I have discovered the answer and it is brilliant.
Snape is Draco's father.
It all fits! It's why Narcissa went to Snape knowing he would help! It's why Snape agreed even though he's on Dumbledore's side! It's why Snape killed Dumbledore even though he didn't want to! All to protect his son.
I am infinitely clever.