Stealing the last piece of food from a starving child goes way beyond mean. It's despicable — a vile and harmful act.
Mean, nasty, and scummy are synonyms of despicable, but they don't fully convey the awfulness of despicable. A man who lies to get a promotion meant for his co-worker would be considered mean. But the despicable man — the one who is so awful and worthless that he deserves to be despised — is the one who burns down the office building and frames his co-worker for it.