Golding chooses to have the boys encounter a piglet, which is significant for a few reasons. First, as a piglet and not a grown adult pig, we can draw parallels between the animal and the boys as young innocents. Second, since the animal is a pig (as opposed to a deer), we can make a connection to the character called Piggy. This is a moment of foreshadowing.
Let's look at the boys' encounter with the piglet:
They found a piglet caught in a curtain of creepers, throwing itself at the elastic traces in all the madness of extreme terror. Its voice was thin, needle-sharp and insistent; The three boys rushed forward and Jack drew his knife again with a flourish.
Jack's initial reaction to seeing the animal is to pull out a knife and attack the helpless animal. This instinct would support your identified theme. When Jack thinks, he pauses. He does take a moment to consider what he is about to do. This gives the piglet enough time to escape.
Your theme is also supported by Jack's attempt to explain why he pauses to the other boys:
"I was choosing a place," said Jack. "I was just waiting for a moment to decide where to stab him."
Jack does not admit the truth, even though Golding tells us:
They knew very well why he hadn't: because of the enormity of the knife descending and cutting into living flesh; because of the unbearable blood.
That would be admitting to humanity, and instead, Jack chooses to be cruel. He continues to insist,
"I was choosing a place. Next time—!"
He snatched his knife out of the sheath and slammed it into a tree trunk. Next time there would be no mercy. He looked round fiercely, daring them to contradict.
Jack will not show mercy to a vulnerable creature in the future. Additionally, the fact that he must defend himself to the boys demonstrates the cruelty of the whole group. None of them will admit what Golding says: "they knew [it] very well" to be "unbearable."