Quit framing the war as the cause. From the article:
A spokesperson for the UN in New York sent the Guardian a statement about Mokhiber, saying: “I can confirm that he is retiring today. He informed the UN in March 2023 of his upcoming retirement, which takes effect tomorrow. The views in his letter made public today are his personal views.”
So he informed the UN in March 2023 that he wanted to step down and now he did it.