Gale literally betrayed the goddess of magic...in terms of the most likely to try to betray you and kill I'll put him at the top of the list.
IIRC, he did not go into detail about what exactly happened, so....
...did I miss a conversation or conversation option? ...is this non-EA knowledge? ...is this your interpretation?
Larian website says he tried to usurp her power. He doesn’t tell you that, but that might imply he’s lying by omission.
So he wanted to become god in place of the goddess? Yes, one imagines she'd be somewhat pissed about that. It is of course unsurprising that he doesn't tell you that, I just wondered where that knowledge was coming from.