The constant abuse under the mountain made her feel weak mentally, then after being allowed to sit in the sun on Scarlet's tree she realizes that's all she wants after years of abuse. A chance to stay where she feels safe. But the dragonets end up attempting to save her and hurt Scarlet, who because of the peace she brought her, she sees as her savior in a twisted sort of way. She snaps and lashes out at everyone in the crowd as Burn takes Scarlet away. Glory is now not the Dragonets friend, she is now mentally insane and on the loose. Then one by one in her mad search for Scarlet she begins killing the three sisters. First Blaze, the Blister, and Ending with Burn when she breaks Scarlet free. The one who kept her trapped but made her feel peace despite being a horrible person.
Edit: What if Peril and Glory ended up bonding together as queen Scarlet's close dragons.
And if you will, Glory x Peril
Where the Story goes from here I haven't figured out so I'll leave that up to you guys.