I just had an incident where I casted a spell using Agamaggan’s ability and had what I believe to be an unintended effect. Despite being countered, the spell (Shaldrassil) still dealt 7 damage to my opponent. This isn’t the issue since counterspell still takes mana in other scenarios, but what I think IS an issue is that Agamaggan’s effect was still present. Despite 7 damage already being dealt, I could then play a 2nd card also using my opponents health.
I just did essentially the same thing. It was pretty sweet, but I don’t think it was fair. My opponent had 13 health left, a secret (counterspell), and board control. I played Agamaggan and then 8 mana wheel of death. It was countered, but he suffered 8 health damage and I noticed it was letting me play another card at his expense. I played a 5 mana card and he died.