I haven't followed the story that closely because it's too upsetting but I was under the impression that his previous victims were male so I would assume that he'd prefer to keep the boy and get rid of the girl.
His record is all of molesting males, correct.
However, many many molestation cases end up with the victim murdered. Men who prey on young girls...several cases in the news...kidnap, molest and murder them. Likewise, men who prey on young boys often murder them. John Wayne Gacy comes to mind, Jeffrey Dahmer, surely there are many others. Lots of young boys have disappeared, never to be found, or their remains found months or years later. Many cases are never solved, but the victims are presumed lost in the prostitution-porno realm, or dead.
One liklihood is if the boy resisted the pervert, that could have ended up in his death. Or it could be just from the mind of the sicko, that he planned to kill him no matter what.
It's strange, isn't it? Perhaps the little boy fought back like a tiger, and got himself killed in a fight. Maybe the perve drugged him to get him to "cooperate" and gave him an overdose. Why he kept Shasta is a mystery....perhaps he abused her too, but she was docile. The film taken by a convenience store security camera the day before they were caught shows Shasta walking around like a Zombie, arms folded over her chest, looking up into strangers' eyes but saying nothing. She's been terribly traumatized by this freak.