He did what he had to do. The very private thoughts of Severus Snape.
Lament of the Half-Blood Prince
For my first dark betrayals
I've paid and scorned the cost
For if my life was all I risked
Then not much would be lost
No matter one Death Eater's fall
If evil's overcome
But now the price is far too dear
My God, what have I done?
Years I've dwelt at twilight's edge
Poised twixt the dark and light
Held truth close as my shadow
Knew there was no end in sight
And then there came a night-black choice
From which I could not run:
Forsake our cause, or kill my only friend
What have I done?
My oaths to both sides bound me
I could not struggle free
I swore I'd rather die myself
He said that must not be
Without my knife at Dark Lord's back
Our war might not be won
He said, "Keep your vows and live"
But still -- what have I done?
Now Dark Lord's trusted man, I walk
In shadow and alone
And given but one solid chance
I would die to atone
For when I raised my wand and with
Two words pulled down the sun
Lest night should reign forever
My God, what have I done?