Cromwell had a job, and he very possibly did it sustaining the fewest overall casualties. Assuming that you can agree with the cause (overthrow of the monarchy), then he may have done the right thing. If you disagree with the cause, and think the Irish were right in suppoting the monarchy, then no justification can be made.
I think Cromwell was a friend of the Jews. Nevertheless, from a Jewish legalistic perspective, how can overthrowing a monarch be justified in this case (or in general)?