Gale could tolerate all of Jade's antics because of the familial feelings between them. Before he met Swan, he thought he had no bottom line for Jade, since he always let her do whatever she pleased as long as it didn't directly annoy him like when she tried to seduce him many times in the past.
But now that he had Swan, he finally discovered his bottom line; Swan.
He didn't care what Jade did, but if it involved putting Swan in danger, he suddenly had the urge to eliminate all threats, even if she was the last remnant of his old pack.
Nothing in this world matters more than my Swan.
The idea was deeply imprinted in Gale's mind. When he imagined Swan's death by poison, he suddenly lost all the guilt for Jade in his heart.