What does Snowball tell Boxer that shows him to be ruthless?
In Chapter Five of Animal Farm, Boxer accidentally kills a stable-lad during the Battle of the Cowshed and feels instantly remorseful. In response, Snowball tells him:
"War is war. The only good human being is a dead one."
On the one hand, this is Snowball's way of consoling Boxer: he does not want Boxer to feel remorse over a crime which was necessary to prevent the humans from recapturing the farm. Looking deeper, however, this suggests that Snowball is far more ruthless than the reader had anticipated. It shows, for instance, that he is deeply committed to the principles to Animalism, particularly the idea that humans are the enemy, and that he has no qualms about killing a human, should the need arise.
It is also worth noting the contrast between Boxer and Snowball in their attitude towards the lad's death. It is symbolic of the wider divide on the farm, between the pigs and the other animals, which increases as the novel progresses.
When Boxer is upset because he thinks he killed a young stablehand, Snowball tells him to stop worrying because the only good human is a dead human.