This sounds like a classic case of mis-directed aggression. Even though they both went to the vets, the other cat was upset at making the trip and lashed out at the diabetic cat.

I'd suggest that you separate them when you return from a vet trip, no matter if one or both went. Put the aggressive cat in a room after you get back and separate them for a few hours. We do this with our oldest cat for a similar reason and it really makes a difference.