Injun Joe is just about as bad as it gets. He's not simply content with murdering a man. Before he even kills Dr. Robinson, he blackmails the doctor; and then, when the deed is done, he tricks his "friend" Muff Potter into taking the blame. He bears grudges beyond death. Even after the Widow Douglas's husband has died, he seeks to take out his anger on her by mutilating her body.
It's Injun Joe's inability to move on that proves his downfall. He can't stand to leave St. Petersburg without getting his revenge and, so, instead of living the life in Texas, he ends up dying trapped in a cave. Some of the townspeople demonstrate a strange compassion for Injun Joe after his death – and Tom can sympathize when he sees Joe dead at the entrance to the cave, worn out from a fruitless struggle – but he is most definitely a villain.