It can't be right for socialists to judge historic figures either by the standards of their times or our own. Whose standards anyway?
Surely Marxists should assess great men in the light of historical materialism.
In the 17th century capitalism was the only way forward for humanity.
By defeating the left and right wings of the revolution, Cromwell ensured a fairly smooth transition to a bourgeois regime. Of course the growth of capitalism imposed tremendous suffering on the masses, as the Irish discovered at Cromwell's hands.
But like Napoleon, Lincoln, Lenin and Fidel, Cromwell was one of those rare people whose actions have had a decisive influence on human progress.
You don't have to like him to approve of his statue being in a prominent place.