That's what I meant. Lucy didn't have a radar for sensing out guys with bad motives. That wouldn't matter as much if it was just a one night stand or a brief fling, but Lucy would fall in love. It makes sense when you consider that Lucy must have spent most of her childhood feeling a bit lonely, without her parents around, and not people of her generation who could understand her. Pam moving in changed that a bit. Pam did understand a lot of Lucy's mindset. But those years of a loneliness feeling is obviously a big part of Lucy desiring a bloke to look after her. Even her fling with Ray had some of that.
I don't think Lucy ever fell for Roger. My memory is that he came on to her heavy the first time, and she regretted it afterwards. The second time, he showed her the shrine, came on to her even more heavily with more obvious lust involved and she couldn't resist, and then she ended it afterwards, calling their relationship weird. The third time and subsequent times, kidnap and rape, until Bobby and Pam put an end to him. As you say, Roger was a psycho.