In the 2010 movie king John swears on his mother's life that he will sign the charter(Magna Carta). However this oath loses a lot of its importance when you realize that King John DESPISES his mother and probably wouldn' hesitate to have her killed given the chance.
From the 1973 Disney film, during the lines "Reminiscin' this and that and havin' such a good time" in "Oo-De-Lally", Robin Hood and Little John are each urging the other to go first when crossing a river by log. The brilliance? In the original tale, the two meet when Little John prevents Robin Hood from using a log to cross the river; that's what they're remembering.
Also in the 1973 film, Prince John decided to order the execution of Friar Tuck to lure Robin out, a suggestion even Sir Hiss was horrified by. However, there is historical context to this. John and Richard's father, King Henry II, had Thomas à Becket, then Archbishop of Canterbury, assassinated. Where do you think Prince John got the idea?
Also in the Disney film, while Robin is daydreaming while cooking, Little John tries to get his attention by calling increasingly extended forms of his name: first Rob, then Robin, and then Robert, using the French pronunciation (sounds like Ro-bear). It's easy to assume that this is because Little John is a bear in the film and that Ro-bear is a nickname. When you think about it further, the French pronunciation because they live under and speak the language of the Plantagenet dynasty, who are French!
Not to mention that in the legend Robin's birth name is Robert Fitzooth.
"The Phony King of England" is as close as Disney could get to the real-life Bawdy Song "The Bastard King of England" in a family flick; which is similarly a disparaging song aimed at Prince John.
In the 1973 film, the Rabbit family scrimped and saved to give Skippy a birthday present of one farthing - the smallest denomination of currency at that point. That's like having to save up to give someone a penny nowadays. Now imagine the economic hardship that would create that kind of situation.
Maid Marian's death in Robin Hood was already a punch in the guts, but the real kicker comes when you realize that she died in order to save the life of King Richard. The same King Richard who eventually gets back to England, only to bugger off to Normandy and get killed there, giving Prince John the opportunity to ascend the throne and become King. So Marian died for absolutely nothing.
Similarly, every adaptation of the Robin Hood legend which casts Prince John as the ultimate villain because he's "not the true king" runs into the Foregone Conclusion that he's going to end up being the true king anyway.