My theory is that he's sick and tired of all factions and heroes tracking him down and trying to kill him, so he's rooting for us the Outlander to win, to be seen as a galactic hero and a legitimate ruler with a good PR, and then take over our body and impersonate us, somehow faking his death to everyone else who knows he's in us. I really don't think devoring the galaxy was ever his plan, he already told us the Sith Empire was just a distraction so he could have said that just to intensify the conflict and keep his true goals away from everyone's attention. Also it was a pretty damn stupid motives only good for a one dimensional cliche vilain and I think Bioware realized that, hence the reset button with Valkorion instead of Vitiate, same but different. He's already much more interesting when he's not babbling about killing everything.