'Twas brillig, and the slithy toves Did gyre and gimble in the wabe; (1-2)
This doesn't seem much to have anything to do with good and evil, but if we wade through the strange words a little bit, we can see how the poem sets up the scene as something that could, really, be good or evil. Even though in Through The Looking-Glass, Humpty-Dumpty tells us that brillig means late afternoon, the word looks an awful lot like brilliant – something that's nearly always associated with good. But then, right away, we have the word slithy, which is pretty close to being slimy. We don't typically think of slimy things as good things. And what are they doing? Well, they're gyre-ing, and gimble-ing, apparently. Both of these words sound like dance moves, don't they? It's almost contradictory to a word like slithy. And so, because there's no black and white interpretation of the scene here, we get this uneasy sense that there could be both good and bad things going on here.
"Beware the Jabberwock, my son The jaws that bite, the claws that catch! (4-5)
Now here is where we get some real opposition in this poem. The father warns his son about what's out in the woods: the Jabberwock (and also the Jubjub Bird and the Bandersnatch). In this case, our themes of "Good vs. Evil" and "Violence" come together a little bit. We can recognize something as evil (or at the very least, bad/scary) if it physically threatens us. The Jabberwock is being put in opposition to the humans in the poem because the Jabberwock is threatening.
He took his vorpal sword in hand; Long time the manxome foe he sought– (9-10)
And we're off on the hunt to defeat that which threatens us, of course. In classic adventure tales (old and new), the hero doesn't reason with the enemy. He doesn't avoid it, and he doesn't try to convert it. No, he tries to defeat it – and that's exactly what we have here. Directly after the warning is given, our protagonist takes up the sword, because our definition of heroism practically requires him to.