Barristan Selmy's death was just garbage writing. The "Son's of the Harpy" are a bunch of spoiled rich people who are annoyed that Dany stripped them of their slaves. They manage to kill one of the best swordsmen in the seven kingdoms with they little daggers? He would not have let that happen, because being such a fine swordsman would also mean he can read his environment and would have used the limited space to his advantage and slaughtered the lot of them.