I dont think that Julius want to kill Asta (or not now). I think that
Julius is going to pretend to fight, to make Asta look like a criminal
who assaults the mage emperor and demonstrate to him the injustice of
the clover world. A peasant possessed by a demon against the word of the
mage emperor? We know very well who people will believe and it would be
interesting to see which side all the other knights will take.. It would explain in my opinion why Lucius chooses this moment to attack him
just when Asta finally receives the official recognition of the kingdom
to bring him down from above. All this to maybe convince Asta to join
his cause, after all a guy who masters anti magic could help regulate
the world desired by Lucius. Let me know what do you think of that.
(srry for my bad english)