At this point, I can believe either motive, sexual perversion or anger.
If he uses stimulant drugs, is of low mental capacity, and has a history of impulse control, these all lend credence to either possibility.
Maybe Breanne had been shooed out of the backyard swimming pool area before, maybe he looked out, saw her there 'again', and in a moment of meth-fueled rage and poor impulse control, ran out and held her head under the water. Maybe he then couldn't keep her from screaming, so clamped a hand over her mouth and carried her inside. Maybe he put the bike AND her in garbage bags in the back of his truck just to get her out of there. If he is of low mental capacity, I can see him doing this just to keep from getting in trouble, strange as that seems. I have worked with such folks and seen this kind of infantile thinking. If he was also a 'harmless' sociopath who needed strong exterior boundaries and conscience residing outside of self rather than inside where it is for MOST of us (church attendence, regular job, wife and kids w/expectations and consequences), then his wife being gone for a short period might have been enough to let loose a sort episode of possible drug/alcohol use and extremely poor impulse control, whether sexual deviance or rage or both.
I hope he did not have horribly sick fantasies fueled by details released in other crimes, like precious angel Jessica Lunsford. For sadists, those details can be played over and over in their minds until they decide to seize an opportunity to live them out. Hope he is NOT a sexual sadist pervert.
What kind of changes must we, as a society, make to stem this epidemic of sickness? To start, I believe the most base horror flicks should be banned/restricted. When sexually emerging adolescents ( and younger) are at that "imprinting" stage, that stuff can set their sexual arousal patterns toward sadistic violence.
Recognizing the sociopaths and impulse-impaired among us, even though they have yet to commit violent crimes, would be a good step also. You can't arrest anyone until they've committed a crime, but you sure can limit their opportunities to do so.
Bless this family and community, who must all be reeling from shock and whose innocence is now lost forever.