The fear of death follows from the fear of life. A man who lives fully is prepared to die at any time.
The real precision here lies in Twain's reversal: he's not saying brave people conquer death, but rather that the fear of death is merely *symptomatic* of a smaller, quieter failure—the failure to actually inhabit one's own days. Someone who postpones living, who chooses safety over experience or love or meaningful work, has already begun a kind of dying, which makes the final death feel like a catastrophe rather than a conclusion. Consider the difference between someone who delayed that difficult conversation, difficult trip, or difficult career change and then faces a health crisis—the regret compounds the terror. Whereas a person who has said what mattered and done what called to them faces mortality with something closer to completion, not resignation.