For example, when he killed Gareth and Gaheris, he could not admit that it was an accident. With every mistake comes the realization that he is imperfect, and will no longer be able to perform miracles, which just makes him more insecure.
Instead, he put the blame directly on his own shoulders. Another example comes before Mordred seizes control of the kingdom.
Secondly, Lancelot lived a baneful existence as a boy.
He was kept away from all the other children and spent his every waking hour with a fiery old man in a single room, learning to fight, joust, and fence.
Night and day he brooded over his ugliness, his malfeasance.
“The boy thought that there was something wrong with him.
So he went forth to go questing, hoping that the good deeds he would do would balance out his impurity and flagitiousness.
Finally, Lancelot flat out states that the only reason why he performed great deeds was to make up for his troubled soul.
He knew that only the pure of heart could work miracles.
If he could be pure and work miracles, then he would know that any inherent evilness he might have had would be taken away, and he would have nothing to be insecure about.