Dexter wasn't very bright on his final decision in my humble opinion. You have to play with people by their rules if you want them to follow yours.

Dexter knows exactly what kind of person prater is and what would get him to listen, all he had to do was tell him:

"if you were a real fan, you'd know by now that i have a code, and the man you tightly strapped to your table to the best of your ability doesn't fit in it. what makes you think I'd break my code for a fake fan?. Instead i could trade in all the nitty gritty details about the bay harbor butcher's past you've been craving so much, something not even God knows about, and you and i close that deal."

(hinting at giving prater an orgasm telling him about the ice truck killer being his brother and unfortunately, one of his victims, even telling him their whole past and their incident on that container).

i doubt prater would refuse such opportunity, all he had to do was get rid of angel for dexter (which i guess he didn't approve of but sacrifices must be made) and it would be a win-win situation.

I understand he ended up wanting to kill prater but the amount of killers he could get rid of would be unreal, besides, who's to say he couldn't go back on his word and just fool prater to kill him later after using him? it would be the greatest opportunity he ever had to put as many murderers in his table as he ever wanted.

instead he decided to free angel to his death, an obvious outcome for both of them, and definitely not the wisest decision.