He has no previous convictions for rape. He had a very specific behaviour prior to meeting Libby. I think he went out for his usual sicko behaviour and came across Libby. Seeing the state she was in in terms of intoxication and vulnerability, he may have intended to take her to the park and masturbate in front of her. He then realised he could rape her. He was so excited he didn't last long. It was all such a rush of excitement that he forgot about a condom, or didn't use one because he associated condoms with his masturbation as opposed to sex.
A few other thoughts.
I think he manipulated Libby by telling her he was taking her to her mum. She was so confused, she could easily have believed that. No-one else who tried to help mentioned Libby talking about her mum and she refused help from all of them. PR said she wanted her mum and she seems to have willingly got into his car. He told her he was taking her to her mum and if she went willingly she's much easier to control and that means an increase in her speed to the rape site. The green shed he mentions would work as the place he told Libby her mum was.
Regarding possible asphyxiation, he could have stuffed her nose and mouth with snow. She had some cuts to the inside of her mouth which could have occured if he pulled on her lip and pushed snow in with force. He was scratched at this point by Libby. I think she became unconscious and he thought she'd die. I think he may have rolled Libby over so she was face down, before he left, for several reasons. He was concerned Libby's screams may have alerted people, so he left quickly.
Later he either realised his semen is inside her and/or he thinks she might not be dead. If she's found, his semen will be evidence and as a sex offender, the police will come straight to his door. He finds her and she's dead. He dragged Libby to the river and pushes her in. I wouldn't be surprised if he thought any semen would wash out of her whilst she was in the water. People say that water washes dna away. He must have been seriously shocked when told it didn't.
The final visit may be to check Libby's body had gone. It doesn't look like a fast flowing river and he undoubtedly never wanted her found or at least not to be found until the water had done its job in removing evidence.
What's particularly devastating is that raping and murdering Libby doesn't appear to have been satisfying for him. He then went back to his usual perverted behaviour. It beggars belief what a monster he is.
JMO MOO