Draco... you are no assassin.
Read more Professor Albus Dumbledore QuotesFrom: Harry Potter and the Half-Blood Prince (2009)
Severus Snape : Has it ever crossed your brilliant mind that I don't want to do this anymore?Albus Dumbledore : Whether it has or it hasn't is irrelevant; you gave me your word.
Oh, to be young and to feel love's keen sting.
Snape! He trusted you!