The wild deer, wand'ring here & there, Keeps the Human Soul from Care.
Like the last couplet, this one isn't really about animal suffering. While the howling lions and wolves symbolize some sort of power that could free souls from hell, the wandering deer symbolizes the carefree, lighthearted part of people: very chillaxed.
"Care" here means the problems that we have to deal with in daily life—you only have to "take care" if something bad or distressing might happen to you. The free deer in the woods doesn't need to take precautions.
He or she just keeps rolling—like Jeff Bridges in The Big Lebowski. (The deer abides…)
It's also another "world in a grain of sand" moment because the deer is given a pretty mighty task: saving humans from their own tendency to get lost in little worries and difficulties.
The Lamb misus'd breeds public strife And yet forgives the Butcher's Knife.
Ah, now we're back to familiar ground: animal abuse. You might've thought you were finished with that topic.
But you aren't. On the one hand, the lamb is another victim of human violence—and its mistreatment leads to the whole society getting torn by strife. It's karma, poetic justice.
But, here, Blake is also uniting the image of an ordinary, everyday lamb getting slaughtered with the image of Jesus, the Lamb of God, dying for the sins of humankind. This is part of the reason why the lamb asks for the butcher's knife (or the butcher himself) to be forgiven: Jesus asks for his executioners' forgiveness when he's dying on the cross. In Luke 23:34, he says, "Father, forgive them, for they do not know what they are doing."
But there's an interesting contradiction here: even though the lamb forgives the butcher, the crime still affects the society as a whole. On the one hand, there's forgiveness—but there's still a kind of injustice too, since cruelty can only makes things worse.