Why would a female boxer kill an Alaskan husky male dog?

At this point I could say that its because of instinct, the only time I would say this, would be because the female dog felt that its litter was threatened by the other dog.

However, under any normal circumstance (Including the male dog attacking the female) this type of behavior is brought about by neglect from the owner. The owner not properly taking care of the dog (Training, socializing, etc) is probably 99.9% of the problem.