The second part after the comma creates a tone of irony that if you're not willing to to believe in it for someone else, then why should you believe in it for yourself.
The comma would have a longer pause on it and would probably be delivered in a lower and a more calm voice.
The pause would create an impact by leaving the audience to think about it and really ponder the statement made.
When Chomsky uses the word despise, he showing that people generally believe that because they dislike someone, they generally don't believe in something for them because they dislike them.
To show that everyone deserves a fair chance whether you like them or not.