
I've seen dogs becoming aggressive because of both - either the epiphen or the epilepsy itself. I home boarded a dog who was severely epileptic once, the owners told me that without the epi he would fit daily and was odd but not unpleasant, but on the epi, he was randomly very aggressive and would literally attack air. That I witnessed myself, while he was here. He was miserable frankly, but hadn't been on it long.
If there is any possibility that it is the epi then perhaps pexxion might be a better option - as I understand it it reduces stress as a side effect and is considerably kinder to the dog than epi. My friend's collie is on it for stress-induced epilepsy - when he is heading for a seizure he becomes very snappy with other dogs (considerably more than usual, anyway) but the pexxion helps him a lot. His owner is looking at a dose increase after a bad week of build-up this week.
ETA also, an online friend's sprollie has fits and meds weren't that effective, but her vet put him on a big dose of coconut oil (2tbsps a day, I think) and it really helps him. if she forgets to give it to him she notices the change in him almost immediately.