For a fiend warlock durge, would it he fitting to assume Sceleritas Fel is your patron? He shows up to give you gifts, encourages you to commit evil acts, and the powers he gives you like Dark Ones Blessing directly come from killing things, which as a Butler of Bhaal, he encourages, and it works with Slayer form... which I just think is neat.

According to the BG3 Wiki, Sceleritas is a level 12 fiend with over 200hp, which seems powerful enough. Comparatively, Mizorah is also level 12, but only has 75hp.

It all seems like it would fit really well. Daddy sets you up with a powerful fiend to help nurture you to reach your full murderous potential.