This is actually just a slightly cleaned up copypaste job of what I said in the channel. But since the people who would care are asleep or away I am making this threat here.
On the short way from the school to the car I finally found a mostly acceptable justification for not killing! My previous one claimed that no humans must be harmed as everyone has an unlimited potential within them; perhaps they invent fantastic things or change the fate of the world to the better at one point. And as nobody can predict the future, it would thus be wrong to kill anyone and along with them all the chances for a better world they bring along.
However this view had a critical flaw: It could easily be turned around to proclaim that following this logic, animals and mentally disabled humans would thus be worthless as no matter what they could not change the world. Not being able to accept those two things I then dropped that theory.
Until today, where I realised the mistake I made. I limited the potential only to the individual itself, ignoring any effects it may have on its surroundings! Even if someone is born with, say, a down syndrome and stuck at the metal state of an infant all his life, perhaps an encounter with a normal person might change the latter.
It may try to invent a prevention against genetical illnesses. Or a person which takes care of a disabled and perhaps demented great-grandparent of his or hers and begins to realise the value of life or perhaps that he or she should do something to ensure that they're well off when they get old! This could also be maximised to include animals, either pets teaching children about responsibility, or animals eating other animals which may carry diseases, whatever. Plus all living things other than humans uphold the ecosystem.
Of course you could say that killing someone might erase any positive chances the victim had but also making sure it doesn't use any of the chances that would lead to negative results, but if you thought like that you'd have to kill every single human being in the world, so that's stupid anyway. (Even though humanity already is killing itself, heh. :|)
Thoughts?