It’s actually from Paradise Lost by John Milton, Adam says it when he is becoming defiant towards God. It’s quite a sad scene and a generally tragic story, but it’s my favorite piece of literature. I’ve never read Frankenstein but I know it’s quoted on the title page so thought it was fitting, even though I don’t think Milton really thought we were created monsters as much as we become monsters but that’s a different question. Even Satan laments and regrets his defiance and secretly admits to himself that God wasn’t unfair towards him.

I’m not sure if Milton was influenced by a line from the story of the Golem though.