The Right Sector is an illegal battalion that Kiev tolerates because its fighters are well-trained, well-equipped, and willing to take on the separatists. 

There is growing unease in Kiev over the Right Sector, because of Yarosh's call for a vote of no-confidence along with some in the group openly threatening a military coup.

Russia claims (likely accurately), Right Sector neo-Nazis were responsible for the violence in the 2014 Ukrainian Revolution that ousted then-President of Ukraine, Viktor Yanukovych.   

Please consider Fears Grow as Ukraine Rightwing Militia Puts Kiev in its Sights. 

Fears are growing that Right Sector — the only major volunteer battalion Kiev has not yet managed to bring under regular army control — could turn its fire on the new government itself.