---HUGE SPOILER IF YOU HAVE NOT FINISHED THE DARK BROTHERHOOD QUEST-LINE!--------
You should let him become your follower, than kill him in some remote location.
I killed him when he betrayed the Dark Brotherhood, and now his lifeless corpse just lays there in the Torture room after you finish the story line.
The DB was a shell of its former self. Beyond that, Cicero attacked Astrid because of her severe slander against the Night Mother. The DB under her was really a poor excuse of a guild rather then what it used to be, with Cicero really being the only lone follower left. Besides that, Cicero is loyal to
you, the Listener which is the important part.
'Sides, if Cicero had succeeded in his attempt to kill Astrid, maybe the rest of the assassin peeps would still be alive.