Zoro: I don't recall ever describing myself as a bounty hunter.
I went out to sea to search for this one guy,
and I couldn't figure out how to get back home.
Zoro: I didn't have anything else to do, so I hunted pirate ships,
and I made a bit of a living from it... That's all.
He did it out of boredom & to eat, but after joining Luffy, it didn't became necessary anymore