Neither, and Both.
He is a resemblance to a Human, the Chaotic neutral of all moral points - He has the capacity for great misdeed's, such as when greed, revenge or other base emotions are driving his actions.
He has capacity be "Good" - Such as when he cared for the Simpsons in Italy (Albeit, his child tried to murder the family, but that is another character, and not Bob) - But deep, DEEP down - he is just that - Human.
(Or to be 100% literal, a cartoon drawing of a human, with human dynamics in his character plan)
Capable of both 'good' and 'evil', he just chooses 'evil' on a more regular basis, usually because it is funnier, and because that makes it acceptable when he is injured to the viewers (solidifying my point of humans being susceptible to both 'good' and 'evil').