He committed such heinous crimes that he was given a life sentence at Impel Down, and he joined Blackbeard after being freed from the underwater prison.

Source: https://www.cbr.com/one-piece-strong-non-human-devil-fruit-users/

He was the Head Jailer of Impel Down once, however, after he killed those in the prison cells, he was locked in by Magellan.

Source: https://gamerant.com/one-piece-strongest-first-commanders/

Magellan was feared by every inmate in Impel Down, because of his ability to manipulate poison.

Source: https://gamerant.com/one-piece-best-villains/

Pizzaro joined the Blackbeard Pirates after surviving an intense brawl inside Impel Down, which suggests that he is a strong individual.

Source: https://gamerant.com/one-piece-strongest-characters-in-hachinosu/