Attain world peace through being the evil that dominates all. Baku wasn't messing around when he told Yakou that after the abandoned building after all.
I've been thinking about why Baku took ~8 years to reappear after losing StL the first time. Originally I figured it had something to do with the L-file being set up, and him just playing it safe until he could be pretty certain. With this revelation of it always being Baku's goal to behind the "Supreme Evil", maybe it was actually Sadakuni's return that prompted Baku to set his plans for the second StL into motion. He knew Sadakuni would have something to do with an "external evil", being a rebel affiliated with some unknown organization, plus he was the one who got Nowa to set up that match to begin with.
This is building up to either a timeskip out of nowhere, or an open end where Baku sets his sights on something past the Vices. It's sad to see it end; as others have said it before, the discussions and analysis within these comments sections were really enjoyable to take part in, especially when this manga's been finished for almost 2 years now.