Most of the characters in Frankenstein seem to lack compassion entirely. The monster alone shows compassion and kindness, attributes that are soon ruined by the world around him. Frankenstein further questions just how goodness is judged. We are left unable to judge which characters are good, and which are evil. We are left unsure of whom to forgive, who deserves our compassion.
The monster is the true protagonist in the novel because he is the only character who feels compassion, and hence, he is the only character the reader can feel compassion for.