I've always thought both Kano and Ashley Walters were almost on par with each other in terms of acting, but I found Ashley quite weak this series. Not sure if it was because Dushane's storyline was really clumsy and I agree his chemistry with Shelley was absolutely non-existent. There was a lawyer in the 2nd series of Top Boy Summerhouse that he had soooooooo much more chemistry with.
It defo wasn't the best series and had lots of holes and pointless bits but I still really enjoyed it and absolutely loved the ending. I agree it's not really important who killed Sully, there's so many people who had the motive. The message is about the inevitable consequences of living that life.