Well, whether or not it was fresh in her mind is kind of beside the point, because most of what they discuss in the books isn't so much what they happen to be thinking about at the moment (or about a quarter of each Marco book would be about girls, lol), but rather whatever is relevant to the plot of the book. Like, if they have a nightmare, then they'll mention it because it just happened. Or if they're thinking about something that's related to what's going on in the book, they mention those thoughts because they're relevant.
The controller that Cassie may or may not have killed in the first book was no longer relevant by the fourth. So, although it may still have been on her mind, it isn't mentioned. Because there's no reason to 'bring it up.'
And, if she did kill the guy, I think it would almost definitely have been in the heat of battle. For all her hypocrisy, I can't imagine her killing anyone in cold blood, even if she had no other choice.