These outcomes were due to the circumstances that Arthur found himself and also due to bad judgment.
Arthur's poor judgment of people around him caused him a lot of problems. He trusted his wife but she betrayed him by having an affair with Lancelot, a knight he trusted at his service.
His son Mordred also betrayed him by plotting to kill him in order to take over his throne.
On the aspect of circumstance, King Arthur had to fight for his throne. He had to fight to regain his throne. The circumstances that he found himself required him to fight to regain what he lost.
