Why would he become evil by dating her
I think that Lucifer will learn to be good through dating Chloe, however he will do things that are 'bad' to protect her and maybe be more aggressive towards danger, like her job. I don't think Chloe needs someone to protect her , but Lucifer will want to protect her from dangers (e.g. his dad, being shot, etc.)
Think about the prophecy they was speaking on and her being his first love
I don't do this very often but I agree with the others. The question is intriguing though. From what I’ve seen Chloe (however little she understands lucifer at first) does seem to be a positive influence on Lucifer (she being the angel on his shoulder while Mazikeen and later eve are the devils on his other shoulder) to borrow the often-used analogy. Chloe could be the positive influence while Dr. Linda Martin helps him explore his pain, trauma, and so on. Perhaps she’s the means to let finally find peace with what he’s been forced to become instead of perpetually resenting and hating himself for it. Granted his self – hatred did lead him to become much more demonic in appearance (or perhaps that what he really looks when he’s in the Inferno with no need to disguise himself). We’ve already seen how knowing and falling in love with Dr. Martin helped Amenadiel become first a better person than regain his angelhood. But regardless while I doubt Lucifer will ever really abandon his hedonistic lifestyle his relationship with Chloe (whom now that she has seen his fully demonic form can only get more interesting) does seem mutually beneficial even if they never consummate their relationship like Lucifer been badgering her to do ever since they first meet. one other possibility is that Chloe put in Lucifer's path as a redeemer. That is it could be that Chloe is meant to help Lucifer become a fully realized angel again which however much he might despise his father if Amenadiel can fall and then regain his angelhood it’s at least a possibility.
What do you think?