Corporate Philanthropy

Lots of powerful and corrupt individuals whitewash their image by professing to do good works. Even some of the most sensational criminals call themselves "philanthropists" (e.g. Michael "Junk Bond King" Milken.) Wikipedia describes former Seattle Mayor Paul Schell, a notoriously corrupt real estate developer, as a "citizen activist."

So what's the term for the fallacy where a bad person is described as a saint?

I don't think its a fallacy if the claims are true, Michael Milken was both a criminal and a philanthropist, they are not mutually exclusive.

If he's calling himself a philanthropist to advance an argument for him being a good person, you could challenge the argument, charge cherry picking maybe.

It's not a fallacy, but perhaps motivated by a cognitive bias known as the halo effect . If a person does something good that really stands out, bad acts are shadowed by the good. Good critical thinking requires looking at all the facts and evaluating them fairly. As Mike points out, one can be a criminal and a philanthropist, they are not mutually exclusive.