Is it considered injustice for employees to write a letter to the company demanding the dismissal of their manager, whom they described as irritable, insulting to employees, and bringing personal matters into work, and to choose two of themselves to supervise the department instead of him?

There is no harm in filing a complaint and seeking redress from an oppressor with the responsible authority, so that someone else who is fit to manage the work may be appointed in his place. This is based on the Almighty's saying: "Allah does not like the public mention of evil except by one who has been wronged." Ibn Abbas said in its interpretation: "It was revealed concerning a man who wrongs another man, so it is permissible for the wronged person to mention what he was wronged by, without exceeding it."