Oryx did not "betray" the Eucemene.

To betray someone you have to be their ally/friend first. Oryx was always the Eucemene's enemy. He never attempted an alliance with them.

The reason the Eucemene were significant was because the hive couldn't defeat them.

They had soldiers called War Angels that were so devastatingly fearsome that Oryx and his sisters ended up hiding inside their throneworld for a while to avoid being killed by them again.

This was the entire reason he killed Akka and gained the secret of approaching the Deep, to get an advantage that would let him defeat the War Angels. (which ended up being the power to Take)