From 1e:
"Law and good deeds are the meat and drink of paladins. If they ever knowingly perform an act which is chaotic in nature, they must seek a high level (7th or above) cleric of lawful good alignment, confess their sin, and do penance as prescribed by the cleric. If a paladin should ever knowingly and willingly perform on evil act, he or she loses the status of paladinhood immediately and irrevocably."
If you look, all it takes is to knowingly perform a chaotic act. If you knowingly do something, you are making a willful(not necessarily willing) choice to do it. If all it took was willful, then Gary would not have adding willing in addition to knowing with regard to evil acts, since knowing already encompasses willful(choice).
2e says the same thing, but adds that an enchanted or mind controlled paladin that does evil loses paladinhood until he can atone.
3e changed willing to willful, which is a different meaning. However, it also stated that the loss of paladinhood was only until the paladin atoned, so there was no permanent loss of class abilities unless the player wanted it.
4e I have no idea.
5e we are discussing. It went back to having to be willing, not willful(choice).