Isn't it obvious? A guard is a blocker and a protector. Goodness, there are so many options! For example, leave two blockers and two mafia killers alive, and then these killers will block each other every night.
The mafia wouldn't do that. There's nothing to argue about.
Comparing this to that statement, the example from Haven is quite relevant: "I'm a Day Guardian, you already know my role, I only have 25%, I won't be able to kill anyone, and until then I'll help the town by voting," - the mafia won, - "oh, I accidentally killed X)".