Is there an ethical difference between the "good" knights and the "bad knights" in "Le Morte d'Arthur"?
There certainly is. While all the knights have valor and bravery, the "good" knights are the ones that hold to a standard of moral behavior. This moral behavior is to give honor always to God. Their fights should be on God's behavlf. The "bad" knights are the ones that give up their moral standards for more personal aims. In other words, they fight for their own honor or for their own goals, and forget their duty to God.
Gawain is obsessed with family honor. He kills Lamorak because Lamorak had an affair with his mother. This murder, done for vengenance, is cited by Gawain himself as the reason for his downfall. It is because he sought vengenance that Mordred is able to seize Arthur's kingdom.
Mordred himself is of course the clearest example of a bad knight. Although brave, he is selfish and power hungry, eager to seize Arthur's crown. This pride is likened to the pride of Lucifer in wanting to take over God's kingdom. Mordred is the villian in the piece and underscores the theme of honor and selflessness by presenting the foil to the virtuous knights.