"Beware the Jabberwock, my son The jaws that bite, the claws that catch! (5-6)
This violence is all in the verbs, here. Most things have jaws, and even little birds have claws, but it's all about what's done with them that counts. Here, the violence isn't happening, but it's posited as imminent or coming. If one were to encounter the Jabberwock, it would catch you and bite you. Because that's what it does. This passage instills fear in the reader because we're scared of bodily harm. Being hurt is no good, and we try to avoid it whenever we can. That caution, combined with the horrific idea of harm coming via something eating us, creates a palpable sense of evil in just two lines.
He took his vorpal sword in hand; (9)
Another "violence is coming!" quote. Here, we have brashness as well. The son doesn't even say anything (that we know of) to the father. He simply picks up his sword. And since we typically don't just brandish weapons around for fun, we assume as readers that his next action is going to be to go hunt down the beast(s) and fight it (or them). So far it's all been implied, but we know something violent is coming. It's just a matter of when.
The Jabberwock, with eyes of flame, Came whiffling through the tulgey wood, And burbled as it came! (14-16)
Lots of nonsense words here, but after some parsing, it's clear that the Jabberwock isn't just taking a stroll. He's whiffling, which if you'll recall from our line-by-line analysis, probably means moving very fast. And "eyes of flame." Here's another threat of bodily harm, although this one is a subtler. We associate fire not only with heat, but also (and perhaps more often) with burning and injury. And since eyes are, to use the cliché, the "windows of the soul," and the Jabberwock's eyes are full of fire. A burning, thrashing soul indeed – the motions here are most certainly violent.