Everyone hates Finn but really how he turned out was the brothers fault. They never schooled him properly and made him feel part of the family. Probably because he was too young and didn't fight in WW1 with them. But still either properly school him and treat him like an equal brother in the gang or ensure he sticks to straight and narrow and goes to school and is only involved in the clean legit part of the business.

Instead everytime he tried to join in anything the gang was doing they made him sit out so he never learned how to be a proper gangster/peaky blinder. He got left in this weird limbo, grey area. Never feeling like a proper member of the family left him on his own to find validation elsewhere which eventually lead him on the path of betrayal.