Without https can't the last relay snoop the traffic?

edit: oh, is the last relay the onion service? So the entire chain is encrypted?

The key used to encrypt traffic is in the URL, everything including path is encrypted from client to the onion service end. What you are saying is true for non-onion HTTP sites, not for onions.

side note: there is a built in flag mechanism called BadExit for Tor that if a relay or exit is detected to be malicious, it is quickly removed from being used.