Use the Restoration Potion Enchanting glitch, make a ring with Fortify One-Handed (Fortify Smithing if you have the Extra Effect Perk then upgrade the blade you want to use) and he'll die after one hit. If your Sneak and Archery are maxed out, you could also probably get him in one shot.
Personally, we never kill Cicero. For one he has a sick sense of humor, and the other is for Role-Playing aspects. Right away you can tell that Astrid isn't pleased that the Dragonborn is Listener (by right the Listener leads the Dark Brotherhood), and she does admit to talking bad about the Night Mother and Cicero. She was probably looking for an excuse to kill him, and making him attack was her way of getting what she wanted. The Keeper (Cicero) is a mandatory member of the Dark Brotherhood, and thus he shouldn't be killed for Astrid's ego. Plus, he's a psychotic jester! Who doesn't want one of those wandering around the house?