That's because he's insane enough to do so. He thought rael left him because he failed to read the letter so he wanted to destroy all the places rael goes to. The emperor is crazily inlove with rael XD. All of them are basically insane but the ending was happy so iz kinda fine for me (。’▽’。)♡
Wait what can someone explain what is happening and what did the emperor kill many people like what's the reason