Probably the high elf trait that gives them higher melee stats at high health kept saving them. The dark riders never even had a chance to trigger murderous mastery, and even if they had gotten to that point they would have already been to crippled to do anything with it.
![Chlöe - Shake (Feat. Jeremih) [Official Visualizer]](http://i.ytimg.com/vi/aDs_1ufpfv8/mqdefault.jpg)
Probably the high elf trait that gives them higher melee stats at high health kept saving them. The dark riders never even had a chance to trigger murderous mastery, and even if they had gotten to that point they would have already been to crippled to do anything with it.