Sylvia got along with Dominator that day because it was a rare period of time where she was having fun in a way that didn't involve inflicting pain on others, which is almost inarguably a component in almost everything she does for fun.
The problem is that Sylvia and Wander are good people who would not stand by Dominator's many awful actions, and would come into opposition with her for this on such a frequent scale that it would be practically impossible for her to grow any affection towards either of them, given her extreme egomania.
The correct, and really only method of reforming her, would be to ride her wave of sadism with apathy or (at most) mild disgust, not really making any vehement objections to when she comes over to your house with brain matter on her shoes or blood on her top, and otherwise be best friend material until eventually her homicidal tendencies decrease over time as she starts finding enjoyment in other aspects of life with her friend.
Even then, they'd also have to be someone that she could like, which automatically disqualifies most people from being a candidate given how antisocial she is. Dominator's a fucking asshole, the amount of criteria that have to fall into place for her to reform on any notable scale is so retardedly specific and long-winded that it reads out like you're trying to fight a secret boss or some shit.