You can warn the merchant that the amulet is cursed (there is a ghost or something which can warn you), but you don't get anything extra for that, and miss the loot he drops.
The 'right' thing to do is kill both assassins, but the game rewards killing the second assassin (the quest entries are updated properly in that case, though I assume killing them both will be fixed when the Larians clean up the other quest log glitches in act 3).
