There's no double standard here. They think that they are the good guys and you are the bad guys. They therefore think that anything that favors their team or harms your team constitutes a good, while everything of the reverse constitutes an evil. Their morality believes in good and evil, and that there are good and evil people in the world, that good must triumph over evil is the basis of their moral standard, they are very much of a simplistic narrative that is commonly found in fictional works.