Why does Mr Tumnus cry to Lucy?

Mr. Tumnus was working for the White Witch and had orders to kidnap any human child that he came across. He invited Lucy to tea and made her fall asleep by playing his Flute so that he could take her to the White Witch. Halfway through his plan, however, he realized that capturing an innocent girl would be wrong and cannot continue to deceive her. He cries out of shame of what he had almost done and out of fear of what the Witch will do to punish him if she finds he did not obey her orders.