Additional things to consider:
People act and/or behave differently in different company. On the Dead Souls squad she is the resident creepy girl. At the wedding, she is guilt driven scotch guzzling wedding ruinier trying to help as best she can. Plus cake.
An example that encapsulates what kind of complex woman Illyana is: She is a totally different person with Kitty. She also behaves differently with her original NM crew. Etc etc.