I've been giving this question a lot of (probably too much) thought.
I return to Ep3, when Anakin embraces the dark side. His motivations are: firstly,
"Learn to know the dark side of the force, and you will be able to save your wofe from certain death," - Palpatine
Anakin's initial seduction towards power was motivated by a desire to protect those he loved from death. To be strong enough to save them.
Again, at the crucial deciding moment ("the Turn"), Palpatine, (himself a longtime mentor to Anakin) appeals that (only) he "has the power to save the ones [Anakin] love[s]" while utilizing force lightning for the first time in the films.
This moment brings me to Anakin's second passion: democracy. As Palpatine fades, Windu decides that "he's too dangerous," and that "he [Palpatine] has control of both the Senate and the courts."
Anakin simply states that "he must be allowed to stand trial." Anakin ends the engage with Windu shouting "I need him!" and gives a hand slice to Windu as he was winding his saber back for the kill-stroke.
During his pledge of alliegance, Anakin confesses that he will "do whatever you want; so long as you teach [me] how to save Padme."
Then, as a secondary point and motivation, we should consider Anakin's first directive after being christened 'Vader' : to protect "the senate" ("democracy") from the certain wrath of the Jedi. Vader agrees, saying "I agree. The Senate will be the Jedi's first target."
Based on the exchanges during and before "the Turn," it would seem that Anakin/Vader's two great unfinished projects were preventing death (or preserving life?) and protecting democracy.
Vader echoes both of these desires on Mustafar:
"Love won't save you, Padme. Only my new powers can do that," and then pleading "don't you see? I have brought peace to the Republic. I am more powerful than the Chancellor; I can overthrow him!"
Moments later, to Obi Wan, after force choking Padme unconscious:
"I do not fear the dark side as you do. I have brought peace, freedom, security, and justice to my new empire!" But rather than aligning himself with democracy like Obi Wan, Anakin declares "if you're not with me, then you're my enemy."
I think that, by this point, Anakin was too deep in the emperor's cool aid to know what was going on.
In the novelization of TFA, Leia muses early on about the general disorder and "fear to independently govern" that many systems faced after the Empire fell.
In my mind, one of these is a lofty political task, and the other is a lofty force-user task.
Further, absent a formal Jedi council, there doesn't appear to be a need to separate a Jedi from an orderly or "autocratic" political agenda so long as it "walked the line" (?)
If Kylo Ren is continuing a darkside task and is afraid he'll never be strong enough, then that task must be to protect someone he loves from death.
I note Rey's flashback, where baby-Rey is about to be stabbed in the back by Raiden, only to turn and see Raiden killed by Ren as he wound back to strike her.
The , Rey was marooned on Jakku by someone flying Ren's ship before it got wings and the jet black paint job.