This post is prompted by a remark in another place by someone who thinks Richard III was a tyrant; it fits the chronology of this blog because historians often refer to Richard II's period of reign between 1397 and 1399 as his 'tyranny.'
What does this actually mean? Medieval monarchs were all dictators. In England they were limited only as far as custom and Parliament could bind them. It isn't sensible to expect them to behave with the scrupulous regard for the law exhibited by modern elected officials. (Pause for hollow laughter.)
I would argue that no medieval English monarch had the technology or staff available to be a 'tyrant' in the sense that Hitler or Stalin were tyrants. Medieval government worked largely through the voluntary effort of local gentry, or it didn't work at all. The king could command what he liked, but he didn't have a standing army, a police force or a large supply of bureaucrats to enforce it. (Well, Richard II did have a sort of standing army but it was really more of a body guard. Similar to the Yeoman of the Guard started up by that nice un-tyrannical Henry VII.) So even the worst oppression had moderating influences if the king did not have enough support down the food chain.
The medieval definition of a tyrant was a ruler who reigned for his own benefit rather than for that of the community - the community in this sense being principally the nobles, gentry and other powerful men. If you want a working example, check out the Viscontis of Milan. Now they were tyrants - you could be executed just for walking through their private garden. No Plantagenet comes close, in fact they were pussy cats by comparison. Hell, most of the Tudors were pussy cats by comparison.
'Tyrant' is a word that gets passed around quite a lot - often it means no more than 'authoritative ruler I don't like.' Some people describe President Lincoln as a tyrant - when you consider his extra-judicial imprisonment of sundry Maryland legislaters and various newspaper editors, you can see where they are coming from. The defence for Lincoln is 'necessity'. It's an old tradition that in exceptional times, rulers may use exceptional measures. This was well-established in England - as late as the 17th Century a court of law held that only the Sovereign could judge what was 'necessary' when the realm was under threat. It certainly was OK to temporarily appropriate the property of subjects and impose special taxation. (It took a civil war to take some of these things out of the Prerogative.)
In the case of the third Richard, England was faced with war with France, Brittany or Scotland for all or most of his reign. He also faced internal rebellion. In the circumstances, he was entitled to use what we would now call 'emergency powers'. His reign was far too short for us to know how he would have behaved under normal circumstances, as opposed to clear and present danger. I suspect, at the lowest, no worse than Edward IV or Henry VII.
As for Richard II - well, most of what he did was under the authority of Parliament. If he was a tyrant, so were the Appellants when they used the Parliament for their ends. What was dodgy was the arbitrary life banishment of Bolingbroke and seizure of the Lancastrian inheritance. The threat to property implicit in this decision scared the hell out of the landed classes and this is certainly one of the issues that tipped the scales against the king.