I own nothing
It came with the job. The same way you expect over time or unfair work hours, betrayal was part of the job. But that didn't mean people took it well.
Some of the Shadows were working with the Mad Hatter and other such as some of the Ice Villains. The plot went wrong. Hatter got caught. As they were taking him away he cried for help. The assassins had promised to help, get him out. The ice villains promised to prevent anything from happening. They never did.
"You promised. We had a deal!" Hatter screamed as he was led away in cuffs to Arkham. The Shadows merely sunk in deeper and disappeared. The ice villains followed. He swore his revenge.
The others thought little of it. They were lethally trained. Super powered. They had nothing to fear. If Hatter came after them they dealt with these direct approaches basically on a daily basis.
But they didn't expect him to not attack them head on. They didn't expect him to attack them indirectly, but also straight to the heart, because even shadows have a heart. Even frozen hearts shatter. They didn't expect him to get out so soon. They didn't expect an attack on children.