The First Order isn't kind to Omegas, and Hux has fought his entire life to gain his current rank.

Kylo Ren suddenly understands a great deal about General Hux's personality and attitude when he sees the redhead fighting off an Alpha who thinks Hux's natural station in life is on his knees.

Hux, in turn, hates Ren because he knows that if the Alpha pressed his claim, there would be nothing Hux could do to stop him.