So its basically [I think] saying if you were to treat a dog with pity and show no shelter of compassion then you will also treat humans the same way.
Metaphors and similes are used to add depth and meaning to the poem, so it looks normal on the surface but when you re-read it you uncover a deeper meaning :)
The best words to describe the tone of "Samuel's Memory" would be sad and angry.
At the very beginning of the novel, he is curious and has a thirst for knowledge, but half-way through he goes through a state of despair where he feels that nothing can go right for him again.