One had to know that when Mo Na emperor sent over the mail, Luo Feng had already suspected it was connected to Purple Clock emperor. Now that he knew about the news of the tower being public, he could almost confirm it was him. This conclusion was also based on Purple Clock's insane character.
"Purple Clock, I haven't begun to search for you and you've already started looking for trouble." Luo Feng cursed within, "Looks like you really want to kill me!"
"You've got guts."
"You've really got guts, you aren't afraid that I'd share the news of your ancient clock."
Actually when they left the tower, Purple Clock had thought hard and understood one thing. If the human had no way to deal with him, he wouldn't let it rest. It was very likely the human would spread the news of his ancient clock, and since the human would fan the flames of his treasures, it was better to force him out early, killing him was the right thing to do.