I think the problem here is that "the Absolute" as a faction... isn't really anything. The Absolutists are all mind-controlled dupes. The only side you can be on is serving the chosen. The Absolute isn't even meant to be a thing that lasts, they're a tool that if I understand correctlly, will be thrown aside once the plan is done. Being on their side means being a disposable puppet, nothing more. Being a true soul means being a disposable puppet, that's all there is to embrace. You have to be pretending to be on their side because the only real logical options are you want to stop their plan or usurp their plan. Everything the chosen promises their followers is a lie, a fabrication for a plot that has nothing to do with anything presented. And as much as people like to tout "Freedom of Choice" if you want to be a servant of the absolute, then your character would have to abandon the prism and become a mind-controlled follower and end the game.
That is all true, though all those revelations regarding the nature of 'the Absolute' and its role in the chosen's plans are not information you have beforehand. I think you can still justify your evil character initially taking that path and eventually coming to the realization that 'serving the Absolute' means partaking in the plot of the chosen, either by helping it succeed or by usurping them. This is enabled by having the prism in your possession, allowing you to freely pursue your own agenda. It could even be used to fuel the Dark Urge quest to reclaim their seat as Bhaal's chosen.
