Oda said he thought half the Supernovas would drop out of tough New World already. It’s survival of the fittest. Zoro’s drive won’t allow him to stop until he’s the “Strongest Swordsman in the World”, so it was inevitable that even a beast like Scratchmen Apoo is taken out.
In Skypiea, Zoro told Ohm he prefers not to fight with a motive. Personal rage would stifle his growth as a swordsman. That changed after Ohm hurt Chopper.
In the Wano arc:
1) Zoro was enraged by food wasting. He related to O-Tama and the starved Wano citizens, probably drawing on his own experience with starvation.
2) He wants to avenge Yasu.
3) He was angered by Kiku’s amputation.
4) He will most likely discover he’s a descendant of Wano’s Shimotsuki Clan.