Just because you don't like a feature, doesn't mean it's a bug. GMs tend to find way to prevent full-on murderhobo players.
That's one of them.
It doesn't matter that he is in your party or not. That just makes him an Npc instead of a companion. Npcs can have an effect on the game too.
It's not his magic, again, it a condition that he has.
It's unique to him. The game could convey that in a better manner but that just how it is.
Gale does give your some warning and what happens after he dies is fairly explicit and part of the story.
Edit : it's not a bug, but feel free to make a suggestion if you want :
https://forums.larian.com/ubbthreads.php?ubb=postlist&Board=87&page=1
