Kind of felt bad for Inga

As she kind of was mistreated by Margaret. And Gray Baker spreading rumors about her and Roman having an affair. Plus, Inga was only trying to make a living for her and her son.

IMHO, think that Frankie killed Margaret over his mother's treatment by Margaret. Plus, Roman could never love another woman like Margaret.
So he killed her in order to also, solve his mom's problems. In addition, to his hatred of Margaret.