It was over-dramatic to some, to hear the sniper say that his boss saved his life. To others...others being his older children and brother...it was a little bit hard hitting. That despite Severin's tries to make his brother happy, and his children just being there, that a criminal mastermind who (probably) didn't give a damn about Sebastian made him feel happy.

To Sebastian, the only one knowing the extremes of his own feelings, it was simply that the stable life and happiness he was given was what saved his life. The will to quit drinking just for his boss, though that rose up years later. Happiness and a sense of stability: two things Sebastian never felt at the same time.

But even with his odd source of joy, Sebastian could still feel sad, as many with what he's been through may. But now he had a safer cure: his boss's voice.
Replaying old orders through the dark smog called memories, listening and truly focusing on the voice that bossed him around, the Irish lilt that lulled him into safety.
Just another way that Moriarty saved his life, in a much more dramatic way.

Severin, while not too fond of Moriarty, had to admit to being thankful for the hiring. Seeing his brother happy was all that mattered to him, after all. Especially since he had to watch his brother in misery for years.