The fleet admiral knows some shit about void century. Just remember Sengoku reaction during WB speech. He clearly knew what WB was talking about. WB even say "Sengoku, you and the government are afraid..."
I don't know, I always interpreted that reaction to just be about losing the balance in the world, hence the "Great War". At that point in the story the biggest focus was on maintaining this balance, and hoping that the Yonkou kept each other in check in a way. Which, at that point when WB confirmed One Piece existed to the world meant that pirates all over the world got their motivation renewed, plus they lost one of those four keeping each other in check.
Nothing about Sengoku's portrayal has ever struck me as "evil", especially given he was Garp's partner, and aside from the dubious ignoring of what went on in God Valley (which I think was just Oda being trash at writing) I doubt that Garp is a bad guy either considering his relationship to Luffy. Would Sengoku know all of this and keep it secret from Garp somehow? Did Garp know it then too? That seems odd to me.