I don't even understand his "complex" motivations. It seems more like a bunch of plots were fused together in a Frankenstein monster, then someone gave it a hat, a steampunk beak mask and taught him some latin sentences.
I liked Delta as a character because almost everything he does and why is left up for interpretation.
One of the reasons was for him and Phi to be born with powers. Thus he had to put Diana and Sigma through a life or death situation or else they might have been born normal.
The other reason was to possibly release Radical 6 into the world and kill a terrorist who would cause humanity extinction.
Last was that if Radical-6 was not released, he would pass the task of finding the terrorist to the nine survivors of the game.
My own personal theory is that Delta along with Old Phi used their powers to see the future and maybe try to stop humanity extinction, but they fail. Delta, in a desperate last resort, develops the Radical-6 virus in hope to kill the terrorist and leave part of the population alive (Don't ask me why he needed the whole Decision Game to spread the virus).
If they were born without their powers, the world would end before anyone could do anything about it. Thus the reason they couldn't be born as normal people.
To me, Delta passing the task of stopping the terrorist to the nine survivors means he's quit. That every possibility that he could manipulate and use would, at best, end up becoming the VLR timeline. Thus it's like an all-in bet, all things considered. Either the nine stop the terrorist and maybe create a better timeline where humanity isn't nearly all killed or face extinction.
Another thing I disliked was how consciousness jumping has been rendered mundane. It was supposed to be an incredibly rare power that only Phi and Sigma possessed, and IIRC Junpei and Akane were explicitly stated to be unable to use it in VLR. Now, everybody and their mothers can use it.
This bothered me too.
I would have like to see a bigger difference between morphogenetic field used by Junpei/Akane, the jump used by Phi/Sigma/Carlos/Everyone and Gab and the technological side used by Sean.
It's been a long time since I played 999 and VLR so I might be wrong on this, but the morphogenetic field would be accessing information from other timelines and not jump all your consciousness through space and time. That would mean, for example, in the last decision that Junpei and Akane would have to die, but the other Junpei and Akane would knew everything that happened.
In the same vein, Sigma/Phi jumping power allows them to change bodies and see what happens in other timelines. However, if those versions of them die, the knowledge isn't accessible to other Sigma/Phi. The jumping power allows them to escape death, but in order to learn, they have to go through everything themselves.
Sean, being a quantum machine can, theoretically, jump between timelines and have a "pool of memories" just like the morphogenetic field. It could be extremely OP thinking like that, so I have no idea how it could be used.