To sorta agree with the above, you folks who are saying that there are races that are evil, and thus deserve to be killed, could be viewed by some as being evil yourselves. You would then need to be killed.
In my games, anything that is intelligent has the free will to choose how it acts. If a creature is just supposed to be a monster to be killed, then I don't make it be sentient. There are many monsters created by sorcerers and gods and such for special purposes, but these things do not have free will. They only act as they are directed to, and short of supremely powerful magic to grant them a soul, they are only monsters.
I guess that's what it comes down to. A soul gives you free will to choose. Otherwise, you are just a puppet of a greater power, be that a sentient who created you, or just the random forces of nature.
First of all, in my games, demons and devils don't exist as they do in D&D. There are malevolent energies that gather in certain planes because they feed off the evil of sentient creatures, and magic-users can draw that power into our world to create things that are equivalent to demons, but there are not cultures and ecologies of demons on other planes. Such beings only have corporeal form in this world, and in that form they are a manifestation of their creator, and have no soul.
Likewise for angels, being drawn from realms empowered by generosity and virtue. Gods, on the other hand, are sentient beings, and supremely powerful ones, and often they create servants and beasts to do their will. These things are not people, though. They are tools, created.
Of course, then there comes the ambiguity: what has a soul? How do we know? Is the monster that says it has a soul truthful, or was it just made to think that, and it really it simply a tool?
Again, most warriors won't care to worry about these issues. They're neutral. But some will care, and they will try not to harm beings that are not an immediate threat, and to help those who might become a threat if left in their current circumstances. Those people are good.