The difference between mercenaries and missionaries, may seem like opposites in most of every way, yet, the difference may be smaller still, perhaps even blurred.
The Mercenary puts targets out of misery. They are paid to end, or prevent, the misery that their souls would or have expensed.
The Missionary aims to convert others. Under a cloak of religious acts, they seek to take those traveling on weed-filled paths, and move them to the sand beneath their sun. But when a closer look is done, it appears they just want company to share misery upon.
I suppose Assassins can be dissected too. Yet they are more like Missionaries than Mercenaries can ever close to come. Despite the pay, they truly believe in the righteousness of what the do and what they’ve done.