Nick wasn't completely innocent in all this
He did let her kiss him, he didn't try and stop her and make it clear he's not interested in her. He also got out of the shower naked with his bathroom door open while she was there. He was clearly spying on her from his window and looking at her body while she was in that bikini sunbathing.
He then snuck into her house, went into her room and pulled a pair of her panties from under her bed with his feet and looked at them. And he was clearly not shying away from looking at her naked when he was in the closet.
He just kept doing things to make her think he was interested in her, she knew he was lusting after her and he was. Then he tried to act all innocent with the cops like she was harassing him since he moved in, well bud maybe cause you did lead her on a bit, he definitely didn't turn her away early on when she kissed him and was teasing him in the bikini.
I'm not saying she was innocent either, i know she was crazy, i'm just saying Nick isn't as innocent as he was trying to act with the cops. He was clearly turned on by her to some extent and loved the fact that she was lusting over him you could tell, when Amy told him she has a crush on him he smiled and his cheeks got all red "haha you're crazy".
He didn't wanna turn Adrienne away early on cause he liked it and didn't want her to stop, up until he realized she had a screw loose and was psychotically obsessed with him then he was ready to ditch her.