Bug with Siding with Menzzango for an Undead army?

[Drirlake]

I am doing the side quest where you find an island full of undead and Vampires. I am at the top of the Summit but no matter what I do I could not make a deal with Menzzango the vampire. You can propose a deal to get an undead army and then afterwards you shoot him down anyway which is really stupid. Is this a bug or is there a prerequisite that is hidden? I am playing an evil bleak walker maxed cruel and aggressiveness.