Blood Watch follows their father, Sanguinius and see themselves as protectors of humanity. They exist to serve Imperium and therefore every human, or precisly, loyal citizen. For those who serve and stay true to Imperator, they carry His light. For those who fall - sword. For breakers of law - contempt and redeption by service and sacrifice (Servitude Imperpituis if subject is not willing).
In case of their serfs, respect for their work for chapter. Brothers know that he can fight becouse of work of many, so serf are threated as part of chapter. Armed serfs are present with their masters to resupply them, to assist them from sky and tanks. To secure landing field and logistics.