Sure thing! 

What Psycho Delic wanted with her: 

Psycho Delic hates Roxanne on a personal level because the Temptress has powers similar to one of his own (his aphrodisiac pink smoke), and her powers are much stronger than his. Psycho was one of the main superpowered villains in Metro City before the Temptress showed up and became the city’s most famous, most important, and most effective villain. She showed the city what a real supervillain was. It hurt his pride.

The fact that she’s a woman who uses sexuality as a weapon for her own gain only makes him dislike her more–if you’ve read Overlord, then you know that Psycho’s club is a brothel, and that he keeps the sex workers addicted to his pink smoke, so that they’re more completely under his control.

He wants to get Roxanne under the power of, and addicted to, the aphrodisiac smoke, so that he can keep her under his control–she has so much power, and he wants to use that power for himself. To make her use it in his clubs, to make her pull heists and give him the loot, things like that. And he also just wants to have power over her simply so he can feel powerful.

When he uses the aphrodisiac smoke on her and says “I can give you what you want, beautiful”, he’s trying to make her have sex with him, but it’s not really about him being sexually attracted to her, it’s a particularly nasty way of him wanting to have power over her. 

Especially since Roxanne uses her powers to make people want her, but doesn’t ever actually have sex with any of them. Being the one who finally ‘got’ the Temptress would really be something Psycho could brag about.

Why Roxanne agreed to meet:

Roxanne doesn’t really interact with people socially very much at all. She doesn’t have a very good understanding of what kind of person Psycho Delic is. And she’s been so very lonely for so long. So when one of the city’s other villains asks her to meet so that they can talk about teaming up, she honestly wants to–not because she thinks it’ll make her a more effective villain, but because she just wants to talk to someone, work with someone. He’s got superpowers similar to hers, too, so maybe she won’t have to feel like such a freak.

And what if he’s planning on killing Megamind, or really hurting him? What if that’s what he wants to talk about? It’d be much better to know, in that case, so she can figure out how to stop him ahead of time.

Why she didn’t want Megamind to know why she killed Psycho:

A big part of it is internalized victim-blaming. Like so many victims of sexual assault, Roxanne feels like it must have somehow been her fault that it happened. Like it means she somehow deserved it.

And she’s ashamed of it having happened to her; she feels like it means she’s weak, too. Megamind is her nemesis, how could she tell him ‘I’m so weak that someone hurt me like this’?

(and of course it’s never the victim’s fault, and being attacked like that doesn’t mean you’re a weak person. but it is so common for survivors of this kind of thing to struggle with feeling like this.)

It doesn’t help that Roxanne already hates herself, either. She already feels weak, already sees herself as evil. She says she’s not sorry for killing Psycho, but that’s because she knows, objectively, that he would have hurt more people if he had lived. Emotionally, she’s horrified at herself, horrified at having killed him, at being the kind of person who kills people.

But Megamind has never seemed to see her like that, has always seemed to see her as not completely abhorrent. 

Having the good opinion of the one person who she can’t artificially influence towards liking her–it means so much to her. It means everything to her, really. Especially since Megamind is such a good person. If he can see her as not completely evil…then maybe there’s something good in her after all.

So she doesn’t want to give Megamind a reason, any reason, for her killing Psycho Delic, because she’s actively trying to get Megamind to hate her at that point. As self-punishment. She feels she doesn’t deserve his good opinion, doesn’t deserve his kindness. 

So she tries to provoke Megamind into hating her, into hurting her emotionally, because she feels that she deserves to be hated and hurt. 

And she knows that, because she cares about Megamind so much, being hated by him will hurt so much more than being hated by everyone else.