Rehashing some thoughts that others have brought up previously:
Maybe TS' "truth" was that she didn't want to be in a relationship that "no longer served her"? Her sister said she spoke the truth, and some people don't like that. In a video, she said something about why do "we" hold onto relationships that no longer serve us.
Rejection is extremely powerful - on par with money or love for a motive. Perhaps TS was returning early from the trip to move out of the house while the girls were not there. MS was the primary caretaker parent, so it would make sense for him to stay in the home. Leaving MS and the girls with her family would reassure the girls that they were still all a family. TS seems like someone who would try to put the children first and do things in an open, responsible way if she felt she needed to leave the relationship.
Perhaps MS sent his friend to try to talk her out of it, and murder by JR was the backup plan. MS had bragged about TS to no end, according to Skinner, so maybe the blow to his ego in front of family and friends (including daughters) was worse to bear than the guilt of murder.
All of this is just speculation on my part about what could possibly have resulted in this terrible situation. JMO.