That's exactly what I was going to say. He is quite competent within his own strange belief system.
But whether prison itself would do him any good; I don't know. Do some offer BOTH incarceration and mental health treatment? If he abused her, which I assume he did, he needs to be locked up for a good while; AND treated, if possible IMO.
I don't know that much about this case, but just a little reading, so I really have no idea what we should do with people who think they are somehow serving their "God" by doing bad things. What happened to the Jeffs fellow, and the people of that weird Texas cult? It's gotten really quiet.
In my opinion, Prison doesn't do anyone any good. He needs to be in prison to protect society. I believe as many others, if he was competent enough to kidnap her out of her bedroom and keep her hidden all those months, he's competent to stand trial.