We are going to put Meursault on trial for being evil. What does this mean

This is a most interesting idea. To Camus, the whole idea of "good" and "evil" was absurd. In his existentialist mind, good and evil are human concepts meant ease our angst over living. Meursault would not recognize the label of "evil". Meursault would argue that as the universe is indifferent to him, he is indifferent to the universe. When somebody dies, the world goes on. One can label him anything they want...good, evil, callous, indifferent....none of these labels matter. Because every living thing eventually dies, everything else is an illusion. This is a pretty heavy topic but I hope that helps you a little.