Perhaps, he didn't want the cops to question her that night in fear that Alaina would end up saying something about him being there in the woods as well at the time Ariadne died. And then she also knew about his torn clothes too.
What if he feared that if the cops got a chance to talk to her it would lessen their suspicion on her and then they would start looking at other people; someone who actually did it, someone that probably was him.
Alaina backed up against the lockers with a thud as the revelation was way too big for her to not be unnerved by it.
So did he save her to not really save her but to stop her from telling her truth and risking people actually believing her? And maybe that was the reason he was afraid to be seen with her and that's why he blamed it to be her fault that the detective took him with them.
