Vito is nuanced, but only seems like a good guy if you're not paying much attention.

In his very first scene, you see him humiliate Bonasera merely because Bonasera "sided" with legitimate society over underworld connections.

From there we see one horribility after another carried out at his behest. We also get strong hints that he's homophobic, anti-Semitic, sexist, and racist, at least by today's standards.

In GFII we see that his rise was predicated on the killing of Fanucci, who makes no threats of violence, merely so Vito can keep an acceptable profit margin on his thefts.

That's not even getting into the book, where his support of Luca Brasi alone makes him 100x worse.