What do these two sentence mean?

“…they fell upon him without mercy, xxx leading the pack.”

“…in the contempt which they all knew he felt for them, and which he seemingly made not the least effort to conceal.”

Yeah, I get it now. Thank you.

Another one though:

" That to the adolescent is the authentic poetic note and whoever is
the first in his life to strike it, whether Tennyson, Keats,
Swinburne, Housman or another, awakens a passion of imitation
and an affectation which no subsequent refinement or
sophistication of his taste can entirely destroy

I’m having trouble understanding the black part.