So would he be a paladin of a god, or the concept of nature, or?
My first instinctive thought is to not look at this particular character playing into the issue of Nature vs. Civilization. Man can very well be a part of nature, even cities can be - animals that live in communities, hives, herds, etc.
So I would make the focus on the conflict of the Natural vs. Unnatural. Corruption of Nature or Man. Aberrations and Undead. Magic that breaks the cycle (Immortality, undead creation, and if you're militant, then constructs too).
Another issue might be a certain level of natural civility. Just because predators kill and eat, does it mean that gnolls are more in line with nature when they go a-slaughter in a bloodlust.
Depending on how big into ecology your DM is, your character might also desire to "cull" (or even protect) certain monsters which are over or underbreeding. Too many apex predator monsters is bad, but not enough is also bad (because there's nothing eating its prey, leading to overpopulation).
To put a twist on things, maybe your Knight has a good opinion of some Monstersl, because they keep Man in check!