Think upper middle class suburbia kids vs inner city gang kids.
The W's were out of their league dealing with the R's because the R's were aggressive, street fighters, if you will. Rough and tumble men ready to defend home and family with fists or weapons no matter who they had to fight. An example of that would be the fights with other derby members, the confrontation on UHR with IJ and crew.
The R's were out of their league dealing with the W's because the W's were more like the upper middle class kids, nice homes, huge horse ranches, farms. Money in the family. And if my theory is correct, with a willingness to keep that money in the family at all costs. The R's who were by all accounts generous people who would help anyone and by many people's accounts would give you the shirt off their backs, would not have been able to understand that level of greediness and selfishness.
Ergo both families were out of their leagues dealing with the other.
The birth of little S would have brought those two opposites together, added to the fact that JW was pretty certain little K was his also. That gave the R's, who had custody of S and K, and by extension the M's access to some of that money the W's had.
KR was one of the aggressive R's and extremely close to CR1 if everyone is to be believed. With that type character and the people he had to be associating with in selling MJ, he would have came after whoever killed CR1 and his whole family. They may not have felt they were safe from his wrath even in prison. So he had to go. Not so much BR or TR as they don't strike me as the "living life out loud" as RSD1200 put it, type of men. So no need to get rid of them as they weren't perceived as big a threat as KR.
I think they discounted the M's as being someone they could intimidate with money or buy off. But they may have misjudged JM.
I am by no means certain this is the answer, but it could have been a part of it.
