Devadatta was a cousin of the Buddha. There was quite a strong rivalry between the Buddha and Devadatta dating back many lifetimes according to the scriptures. So this rivalry continued even after the Buddha became enlightened, and Devadatta became a monk under the Buddha. Devadatta always wanted to be greater than the Buddha, and so this past jealousy ignited, and he wanted to be the head of the order. He even tried to kill the Buddha. Unfortunately for him and he created a lot of bad Kamma for himself. To my understanding, Devadatta did not become enlightened, even though he thought he was. He had developed some psychic powers, and so he developed a lot of conceit due to those psychic powers and he thought he was greater than the Buddha. But from what I understand, the Buddha did predict that Devadatta would eventually become enlightened. But certainly he may be still working off some of his bad Kamma at present.