Toriel is a caretaker of the RUINS, she is very kind, motherly monster who genuinely cares for any fallen humans who had fallen in the underground, always worries about their safety. She does not let any fallen humans exit the RUINS because of Asgore, he will kill them and take their souls, to break the barrier.

If the human tries to escape the RUINS, Toriel will attack the human and stop him/her. If she kills you, that means she might have accidently killed you, and felt guilt.