Should I Kill Motierre?

This reward is not in the urn when you initially meet Motierre and Rexus. After collecting the reward, the Dragonborn has the option to kill Motierre. Doing so gives the advantage of collecting a few rare gems from his corpse, as well as possibly receiving an inheritance from him.

Should I lie about how much Motierre paid?

The Dragonborn can choose to lie about the actually reward amount, but that is of no consequence. Nazir points out that the money could be used to repair and upgrade the Sanctuary, by talking to Delvin Mallory of the Thieves Guild in Where You Hang Your Enemy's Head….

How do you kill Amaund Motierre?

I killed him by keeping the room to his door open, sneaking near him until I was hidden, then casting Frenzy on him (with the quiet casting perk). He'll either attack you or attack others in the inn, making him killable without a bounty.