I just killed Tarquin even though I knew there was an achievement to keep him alive. Afterwards, I googled him up and found I will miss out on items and xp later in the game. Seriously, he is a total slime bag that poses a serious threat to the security of the Lady Vengeance. At the end of your conversation with him, the party is unanimous in their belief he must be taken down. Other than to game the system, why would anyone let this shit-stain loose on the ship?
If you have no other choice but to kill, you have failed.
Ntwa kgolo ke ya molomo