It's a pun. The painting depicts the assassination of Julius Caesar. One of the most well known of the assassins was Caesar's friend Brutus. In William Shakespeare's play based on the event Caesar's last words are "Et tu, Brute? Then fall, Caesar." His actual last words are probably lost to time, but many people associate the line from the play with the event. As for the pizza thing, it's probably just a setup for the joke.

Julius Caesar likes pizza

Julius Caesar's famous last words were "Et tu, Brute?" (And you, Brutus?) Brutus was supposed to be his good friend, but Brutus betrayed him and was one of the collaborators in his assassination.

Just a play on words with "Et tu Brute?"

Caesar proving why he deserved to get legally executed by a quorum of the senate for treason and poor pizza logistics

Everyone wants more than one slice you cheap ass, and even if they pretend they don't your plan should be to have more than exactly enough so no one goes entirely without.