Only three (3) illegal uses of legally registered NFA Title 2 firearms (machineguns, destructive devices, suppressors, SBS, SBR, and AOW) that we know of...
....and all three murders were done by LAW ENFORCEMENT with their Form 4'd personal toys (not form 10 department guns).
A Miami PD detective used his own Mac 380 SMG to murder a CI who was about to blow the whistle on his corruption in the late 70s/early 80s.
A OK sheriff deputy used his AR-18 to kill a burglar who kept breaking into his garage in the 80s. Neighbor saw him crouching in an ambush on his property.
A MN SWAT officer used his suppressed MP5 to kill his wife, her cop lover, and the internal affairs officer before killing himself in the mid-late 90s.
There, is of course, the most famous one, Christopher Dorner who used suppressors and short barrel rifles (form 4 LEO exempt) in his rampage in 2013.
Want to keep NFA Title 2 guns from murdering people? Don't give them to cops.
All of the data I have seen suggests that concealed carry permit holders offend at far lower rates than law enforcement officers, let alone the general public.
I don’t know of any studies on the subject, but I would be willing to bet that the crime rate among non-LEO legal owners of legal owners of NFA Title 2 firearms is probably within the margin of error of zero.