They should've just left it at Equalizer 2. No meaning to Denzel's character's killing other than to kill. He lost his reason to help those in need. He had many opportunities to "equalize" the wrongs the mafia was doing but didn't.
The mafia as well could've made an example when the boss was trying to find out who killed his brother but instead ran when people were recording them then went home to eat spaghetti. Horrible writing. Waste of time watching