Derek when...he couldn't get to you in time. He knew that the Alphas were going after you to get to him, but he thought he had more time. When he finally got to the loft, Kali dropped your body to the ground before leaving with the others, Derek too heartbroken to notice as Stiles and the others arrived as well. Derek collapsed beside your body, Stiles slowly walking behind him and placing a hand on his shoulder.
'This is all my fault.'