Not only because his current condition can't be said to be good, but he tends to be influenced by Shylla's words saying that his lover can read his mind.
"Axel, do you really think humans can become like us?" Lucifer asked innocently.
A frown on Axel's forehead, of course, represents the confusion that Axel is currently feeling at Lucifer's statement.
"I don't understand sir, can you explain what you mean?"
Lucifer took a deep breath, and then explained to Axel what he meant, as well as explained a bit of the conversation between himself and Jill which Axel certainly didn't hear.
If Axel heard then the young man of course had already been marked by Lucifer.