Why does Indian Joe want to get his revenge on Dr Robinson?

Injun Joe did not really want to kill Dr. Robinson; all he wanted was his money and a little revenge.

Now the cussed thing's ready, Sawbones, and you'll just out with another five, or here she stays."

"That's the talk!" said Injun Joe.

"Look here, what does this mean?" said the doctor. "You required your pay in advance, and I've paid you."

"Five years ago you drove me away from your father's kitchen one night, when I come to ask for something to eat, and you said I warn't there for any good; and when I swore I'd get even with you if it took a hundred years, your father had me jailed for a vagrant. Did you think I'd forget? The Injun blood ain't in me for nothing. And now I've got you, and you got to settle, you know!"

Chapter Nine