1. He isn't a Sith Lord(anymore) after he completed his training because he saw what happened to all Sith Lords throughout history and became more of a Dark Jedi.
2. He is actually a Sith Lord, but Lumiya falsified his origins to seduce Jacen to the Dark Side.
3. Supposedly Darth Vectivus was benevolent, but keep in mind that this was Lumiya’s characterization of the Sith Lord. Darth Vectivus, by way of his holocrons, isn’t exactly a reliable narrator of his career either. After all, even Freedon Nadd and Exar Kun would likely think that the ends justified their means in their power struggles. Palpatine could even say that unifying the known galaxy under his iron fist was a necessary act to resist the Yuuzhan Vong invasion that he had foreseen. He truly believed that his Empire was just and that the murder and terror inherent to his regime were a fair price.
Realistically, I think all claims to benevolence via the dark side are bunk; nothing more than rationalization after the fact. Even Darth Vectivus’ claims regarding ethical behaviour always struck me as the recourse for a relatively weak Dark Lord. He could not take what he wanted directly and so settled for scheming loosely within the the rule of law.
__________________ RealistRacism: "Sheevites, much like the Banites, were meant to increase in power with each member. From Lightsnake to Gideon to Azronger, this was supposed to be the case. However, knowledge must've been lost in some kind of Gravid-like incident, as Az turned out to be a mid-tier debater with a sub-par track record, sh!itting all over Tempest's legacy. Sad."
__________________ RealistRacism: "Sheevites, much like the Banites, were meant to increase in power with each member. From Lightsnake to Gideon to Azronger, this was supposed to be the case. However, knowledge must've been lost in some kind of Gravid-like incident, as Az turned out to be a mid-tier debater with a sub-par track record, sh!itting all over Tempest's legacy. Sad."