On the Ark grieving didn't happen; there wasn't time, life needed to continue on. You knew from practically birth the meaning of 'for the greater good' death almost didn't seem real or have a true need to be grieved. There were some deaths in which the avoidance of the hurt can not be ignored. Marcus thinks of the 320 with each sip of whiskey he took; for them he'd mourn. Half way through the bottle the reality blurs in his mind. Was he grieving for them or what he had become?