Even though Alonzo was greatly weakened in the illusionary world, he was still strong enough to deal with Agalon. He completely suppressed the past Agalon in terms of combat and alchemy.
However, before he could enjoy himself, the real Agalon appeared and informed him that if he ever wanted to leave this world, he'd better go through exactly what had happened in the past, and find the thing that didn't belong. If he didn't do that, the world would fall apart when the time was up, and he would be completely dead…
He hadn't quite enjoyed himself yet, and he was frustrated again. Furthermore, he could've won, but he had to lose. He was so infuriated at his failure, but not so much as with the fact that Agalon came to his rescue.
Agalon came to his rescue, and he had to be crushed by the old Agalon. He was truly of a mind to take it out on Agalon in the illusionary world, even if the cost was death. He could vent his fury anyway.
