Severn - in more complete versions of Long Lankin/Lambkin - Lankin has built the castle in which the lord, lady and baby are dwelling but he has been refused payment by the lord, for various reasons, for his work. Lankin swears revenge and that is why the lord is saying to beware of him. Therefore Lankin is as much sinned against as sinning.
Perhaps the lord misunderstood him when he said that he was a Freemason lol.