In Kid's mind, once Xie Wen the Bounty Hunter learned who he was, he should have wanted to capture him. Even if their earlier "favor" kept him from acting, he would mock Kid's dream like everyone else, giving Kid a reason to attack him.
His reason for wanting to fight Xie Wen was simple, too. Much like Blackbeard's reason for wanting to attack Luffy back then, he wanted to make a name for himself.
Although he had just dealt with Simmons, whose bounty exceeded twenty million, that was nowhere near enough for the proud Kid. A powerhouse like Xie Wen, who could take down over a dozen pirate crews on the Grand Line, was the kind of opponent he wanted to challenge.
As for why he had to make a simple challenge so complicated, besides having only just set sail and not being used to acting like a "real" pirate, he had also just helped those cats, only to suddenly pick a fight with Xie Wen, who was on their side...
This future member of the Supernovas found that a little too embarrassing.
All right, ultimately, it still came down to him not being used to handling things as a pirate.
Yet Xie Wen's reaction caught him off guard. Rather than getting angry or mocking him, he calmly struck up a conversation.
Unwilling to give up, Kid could only bite the bullet and demand, "Aren't you a Bounty Hunter? I'm a pirate!"
By now, Xie Wen had more or less guessed what he was after. Still, Kid had stepped forward to protect his cats. Although there had been no need, Xie Wen appreciated the gesture.
Otherwise... a Paramecia Devil Fruit was the only kind he had yet to collect.
So Xie Wen picked his ear, held his finger in front