sirius and james were smart and good at everything reading into the conversation about them in the three broomsticks in poa just bfore xmas, voldy kills anyone whos likley to get in the way of his takeing over the world
( omg that sounds like dr evil from austin powers )
but there you go anyone that wont join him or is a threat he send his lap dogs (death eaters )to purswade or kill them for not doing what the dark lord wants