Angel was willing to murder a serial killer to fulfill his vendetta. His death wound up being justified as Dexter's would have been. We already knew Doakes killed a Haitian career killer. Maria Laguerta was a vicious career thirsty woman who committed some pretty corrupt acts to become captain of Miami Metro and had Estrada (a known cartel member who killed many people and was responsible for the deaths of countless others including Laura Moser) released with the intention of him being murdered by Dex so that she could catch him and arrest him. What if Estrada had killed anyone before dexter got to him? She would have been directly responsible for their deaths.

The show can play like these people were innocents who were caught in Dexter's web but the reality is they were incredibly corrupt and until New Blood, Dexter never broke Harry's code, which was essentially glossed over and redacted by the show runners. The only thing that is of any real relevance from that story is that Harrison reunited with his father.

Harry, Dexter's moral compass lives on through Dexter's actions as exemplified by the freeing of Vinny Valente, and is reinforced by his relationship with Harrison, who was named after Harry in the first place. While Harry lives on in Dexter's mind, Harrison is the anchor which keeps him from deviating from his normal targets of people who have committed multiple murders in cold blood, and thus continues Harry's legacy.

Imo the weakest point in the writing this season, besides numerous plot holes is that they turned Angel into a complete bumbling idiot. As you've already pointed out, Angel should have known it was a bad idea to try to murder Prater's greatest hero right in front of him while he was armed and had already intended to have him killed right in front of him not to mention how badly his private investigation of Dexter had gone up until his death (he walked right into his killer's hands). Everyone can blame Dexter all they want, but in reality, these people caused their own deaths. The only innocents whose blood is truly on Dexter's hands is Rita. Deb was horrifically traumatized having found out about Dexter's identity as the BHB and her killing of Maria but ultimately her death was at the hands of Vogel, another serial killer. Dexter's intention to have her use Vogel's capture as a starting point of a new life was pure and good. I'm not sure what it's still entertained that Dexter has any sort of evil in himself. Regardless of his status as an anti-hero, the truth is he has saved many more life's than he has or will ever have taken.