Complex sentence: MetalSeadramon decided to go after them himself, he defeated Lillymon and Zudomon after the Digidestined thought they could escape by Sea on Zudomon's back.
Split into simpler sentences: MetalSeadramon decided to go after them himself. He defeated Lillymon and Zudomon after the Digidestined thought they could escape by Sea on Zudomon's back.