From looking at the evidence it seems like Milk used the normal nether portal to get into the nether and sees that there is unID'd body and kills you for that as it is a KOSable reason.
Milk could not have known how long you were in the nether for as you used a different portal to him to get there and was there before him.
It honestly seems like a misunderstanding on both parts and I feel like no action should be taken on Milk.Though I will tell Milk to be a bit more careful.
Perhaps you guys could hash it out on TS and have a laugh about it instead of the hostility?