Debbie was the leader of the group and the most prone to lying in order to evade justice, she had no problem with frequently framing Curtis and Steven so that she could appear totally innocent while still orchestrating the murderous antics of the group.
Both Curtis and Steven seemed to obey Debbie and as such she was the one that dictated most of the murders and who they would kill - though Curtis did sometimes kill without her authority.
She was also unafraid of killing her own blood - indeed one of her first victims would be her own father and later she killed her older sister with an arrow: unlike Curtis or Steven she could at times fake emotion (even if she could not understand it).
In the end Debbie escaped justice at the end of the film and tricked her mother into driving her out of the town, claiming Curtis and Steven were the real culprits - thus as the two boys were arrested Debbie and her mother escaped.
It is then revealed that Debbie is still killing, tricking her mother into thinking she was a sweet little girl but continuing to spread chaos behind her back - it is presumed Debbie continued to kill for the rest of her life (short of being caught, which given how clever and manipulative she was may of never happened).