Was King George a tyrant as he was depicted in the Declaration of Independence?
At least in terms of his dealings with the American colonies, I do not think that it is fair to say that King George was a tyrant. His actions (and those of his government -- it wasn't like he was an autocrat) were really quite understandable in the situation. What I would say is that the king simply was in a bad situation where it would have taken a lot of political insight and courage to solve the problems to everyone's satisfaction.
Basically, the British government expected the colonists to pay taxes. This made a lot of sense but it annoyed the colonists because they had not had to pay these particular kinds of taxes before. Because they were annoyed, they rebelled in relatively small ways (Boston Tea Party, attacks on Stamp Agents). When this was done, the king had to crack down or else he would lose all ability to rule the colonies.
In my mind, his actions were not tyrannical. He wasn't having people executed left and right for no reason. He was simply trying to impose laws on people who had not previously had to obey them. This will never be easy.
In addition to the ideas above, there were some other conditions and factors in the colonies and Britain at the time that contradict the idea that King George III was a tyrant. The popular conception is that the King was horrible, so therefore we revolted.
Other factors/conditions to keep in mind:
1) The people in the British Isles paid approximately 8X higher taxes than those in the 13 colonies
2) The first boycott by colonists occurred in 1763, and the first violence (against the Stamp Act) in 1765. King George did not respond with serious military force for another ten years after that. He was a pretty patient guy
3) He committed $15 million and 40,000 troops as well as the British Navy to defend the colonies (and his business interests) during the French and Indian War.
4) The King allowed colonial legislatures to be elected and local rule to happen much of the time.
So I don't think you can characterize him as a ruthless dictator. In the end, he reacted as any King would to armed rebellion.