If by malice you mean "right now I want to do this
because it will hurt him", I agree that's not how she operates. What Lacey does is "right now I want to do this, if anyone has a problem with it, that's their problem not mine". That's the essence of narcissism. It's practically her response that morning she wanted to go play tennis, but her whole mindset is like that. Maybe without even fully realizing it.
The above is how she concludes her mindfucking to get him to accept. Notice how she says it's all to help him and she won't force anything. Everything is his choice. This is the supposed main motivation for what she's about to do right? To do something for him, not her. And she probably believed that too.
But what happens when she's actually there? He attempts to make that choice and she refuses him. Multiple times. Knowing very well what she's doing. She forces what she wasn't supposed to be forcing. So if her main motivation in the time frame between approaching Damian and eventually getting creamed, was what she was describing above, why did she refuse him the choice? And force him? Isn't the whole thing done for him, not for her? If it's for him, shouldn't he have a say, like she herself told him?
The answer is obvious. Her main motivation in that time frame, had nothing to do with MC. At this point in time, he was irrelevant and he was being treated as such. Her main motivation at some point transitioned from the above, to getting some dick from the attractive co-worker. Who was supposed to be a stranger, but somehow wasn't. If that wasn't the case, she would have picked up the damn phone. It's Occam's razor.