I think it is worth considering that drug abuse may have had a role to play in the murders. For example cocaine overuse can lead to erratic and violent behavior. Also, in some cases, cocaine addicts may experience “cocaine psychosis” in which they become detached from people, activities, and, in some instances, reality.
We know too that methamphetamines can lead to violent behaviour. These guys were taking multiple drugs over a long period. It's well documented that continuous drug use of some types of drugs can lead to violence.
https://recoverycentersofamerica.com/news/what-are-psychological-effects-of-drug-abuse/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4651438/
You make some valid points. However, the broader criminal conspiracy that they planned, the planning of the murders, the relationship MS kept with MM, the relationship DM kept with all of his friends and CN, the bigger picture of DMs life such as the condo sales, his employees who worked at the hanger, his firing of employees all suggest otherwise. MS regardless of his drug could of not taken part on the planning and execution of their plan to murder LB and TB.