I think the problem is the students are giving too much credence to the monster's monologues, but "He is eloquent and persuasive, and once his words had even power over my [Frankenstein's] heart; but trust him not."

All that aside, you can't look past strangling a 4-year-old boy. It's reasonable to call anything that strangles a 4-year-old boy a monster, even if it felt lonely/abandoned.

And even the monster has the self-insight to know that he's fundamentally evil: "I had cast off all feeling, subdued all anguish, to riot in the excess of my despair. Evil thenceforth became my good."