This is a very insightful comment. It also leads to a circular argument of sorts.

IF Carlo was not of such weak character, would he have at least tried to work his way up the ladder rather than feel entitled? But then, IF that was the case, Vito would have trusted him with more responsibility, right?

But what IF Vito misjudged Carlo and he really had the character and shrewdness that could benefit the family if given more power (despite no evidence to support this)? Could such a misjudgement corrupt an otherwise loyal and shrewd person? Or would Carlo have tried to work his way up the ladder in spite of that if he had actually was a man of character?

Ultimately, Vito judged him, by all accounts accurately, but we will never truly know for certain. Certainly, Carlo was not corrupt when he married Connie, but he either was then or later became corruptable. Either way, Carlo's tale ties perfectly into the larger Godfather theme - Every man has a destiny. Carlo's was to become a &^%$!#@ rat.