He is ultimately focused on the light, it is what drives him and how he operates, shifting people in the direction to achieve their goals. Through these actions he could easily buy himself their loyalty. I am just waiting on the day we have to mow down Aldor and Scryers.
Velen voiced doubts, he was dismissed, warnings were there, they just chose to ignore it. If you are manipulated to kill you still bear some responsibility, especially if you get to love it.There was nothing for them to "look into," and both Archimonde and Kil'jaeden thought the essence of Sargeras' offer was genuinely intended (wrongly, it turned out, but all the same). They even appeared to look it over for possible deception, because Kil'jaeden confirms his belief that the offer couldn't have been fabricated. You're basically blaming the victims in this case. Being gullible or tricked doesn't make you complicit in another person's crimes, as it were, excuses or no.