Answer:
In the first movie Gally felt as if the glade was there new home and that they couldn't escape it, he felt as if Thomas was just leading them to a hunch, which he was. And he didn't have anymore hope of getting back. Until he got stung my a Griever which caused him to remember and as a result he knew everything about what Thomas did to them before WICKED put him in. As a result he tried to kill him.
Explanation:
