Spite. Omega Ridley has no feats, unless you count the things he did as Meta Ridley earlier in the game and in the first Prime (easily out-speeded Samus's gunship when they were both heading to Tallon IV, wreaked havoc on the Artifact Temple, and took out an entire squad of Federation Troopers in one attack while on Norion).
Dark Samus knocked Aran out cold for a month, while slowly taking control of the other 3 hunters. She'll probably win, unless she's unable to shoot through Omega Ridley's Phazite armor with her Phazon attacks.