Just.... Why?

Honestly I have so many questions about this reading but the two that stick out the most to me are: Why was Arthur okay with, and honestly kind of supportive, of Lancelot having an affair with Guinevere? And Secondly, If he knew Mordred was going to try to kill him and was plotting against him, why did he listen to him so much? Wouldn't it have just been easier if he had banished him or something?