Why would there need to be another reason for the weapons being as lethal as they could make them? If you want to kill someone then it would be best to kill them totally in one hit. You don't want them to take a round and then shoot you back.

Though it may be more practical to cause massive survivable wounds that force the enemy to expend resources on evac and field hospitals.

To a degree, the lethality of our weapons is held in check by conventions. Some weapons are considered beyond the pale. This is probably because we feel a degree of empathy towards other humans and we worry about what it would be like to be on the receiving end.

The premise of D9 was that the prawns were slaves of another race. This changed late on during development so weapons would have been conceptualized assuming that prawns were slaves and that it would be slaves fighting with and facing those weapons.