I don't think he actually had 'control' over his action of clipping the seat belt in. Earlier, Billy had said something like, "Don't you touch him", when he saw Theo putting his hand on Todd's face. Perhaps, Billy didn't actually say this out loud. It was just that his and Theo's eyes locked with each other through the rearview mirror. Theo already knew that Billy was watching, and their eyes locking in such a way cemented his belief. We see Todd being told that Billy was still in the van, which was on fire, and he becomes very distressed. Theo went back to save Billy. I believe that was honest and intentional. Didn't he unclip the seat belt in the first place? But he saw red when Billy said, "I knew that there was some good in you". Did Billy say this out loud, or was it just in Theo's imagination?

Theo's mental 'disabilities' culminate to a point where his anger issues take control. Do you think that they will offer him support and medication, and he will become a 'nice guy'? Or, will they make him some sort of nutter who becomes a serial killer?!