To me, there are two main things you can point to in order to argue that Cromwell was a tyrant.
First, you can look at his tendency to keep power for himself and to oppress his enemies. For example, he dissolved Parliament when they did not act according to his desires. He also used his standing army to force local governments to act the way he wanted them to act.
Second, I think that you can look at his actions in Ireland. He is criticized for the brutality of the invasion of Ireland, including massacres like the one at Drogheda.
So here you have a person who essentially used military force to close Parliament and to coerce local governments. He also (you can argue) acted brutally towards the Irish. Both of these ideas could be used to argue that he was a tyrant.