To say that Sebastian Moran would do anything for James Moriarty was, frankly, an understatement. He will do anything for that man. Obviously kìll is already on the list; but there's more than just bloodshed.
He would lie, die, steal, break the rules even further than already done so, do any possible gruesome thing to please the mastermind if he so wished. But, as most sides of Sebastian are, this side was hidden too. He didn't let anyone know that, as it could be used against him.
Though, it isn't that particular thing that is targeted.

Asking why he's so loyal is what people do instead. Why be so loyal for someone who would replace you in a heartbeat, they'd ask. Sebastian ignored them, mostly. They didn't know what the criminal mastermind has done, unknowingly, to and for the sniper. People only guess what sort of relationship can be had. But he never listened or cared, unless it was downright insulting his boss. Then there's a problem.
And it never stopped there. Old friends would try and win their gun for hire back, obviously failing. Even actual trusted friends in a tough spot were declined. If their risky business could jeopardize Moriarty, Sebastian refused the offer no matter what. Even if it was a desperate, you're my last chance scenario.

The thought of betraying his boss made him feel sick, the thought of going through with it even more sickening. Let me put it this way: if he only had two choices, one being fight Kali's Kitten and the other doing even the smallest form of betrayal, Sebastian would gladly walk into the tiger den. He was loyal to the point that dying to what he feared was more appealing than never coming in contact with the beast again.