//Whoops.

It would sound insane to hear that a criminal mastermind was the one who made the sniper's life finally feel stable. But then again, maybe it wasn't so insane. Maybe having consistent work and a decent boss was what he considered stable.
But that is only the half of it.

While it took some time to get to that point, Moriarty was the only one who, outside of the good family members, has treated Sebastian as a human being. Previous bosses got him as an employee from a gun to the head, a few cuts to the body. Moriarty didn't do that.
His father never thought anything Sebastian did was something to be proud of, not even his son learning how to shoot from his uncle. Moriarty praised his shooting skills rather highly.

And past all that, Sebastian never knew what a stable life was supposed to be. His childhood and teenage years were both in a constant state of toxic fog, to the point that breathing near the bästard was terrifying. After being attacked from a tiger and unsure of what to do next, his life was still unstable. The pressure of moving out, the stress of being home...it wasn't good.
But then he did find himself in the sort of mercenary business, and even then it was still far from his life today. The risk of getting arrested, the torture he'd get into, the horrible bosses of the past, no consistent income and kids to raise...

But then Moriarty entered his life with a Westwood suit and an offer he couldn't refuse or regret. And then, even at the lower ranks of being his sniper, Sebastian felt stable. He had a consistent income. He wasn't tortured into work. He was far from his father's horrid tongue and fists. He soon wasn't a knight on the chess board.

For once he felt human. He felt happiness. He felt more passion to his job. He felt stable.
And that is why he says it was love at first sight. Because in all the chaoticness that is Moriarty, there was a patch of calm resevered just for Sebastian, unknowingly, and he could see that in those dark eyes.