Cromwell was worse than a tyrant, he was a tyrant who thought he directed by God.
The decision to kill Charles I happened (according to Cromwell) because it was willed by God. Cromwell saw himself as the instrument of his creator and often delayed making decisions because he was looking for "signs from God" to confirm his decisions.
Whatever
I would say he improved the breed but just culling one or two doesn’t really do much.