Thread:Sirkinsella98/@comment-33286111-20181020071621/@comment-33286111-20181020072444

All Megaman wikis online and even official bios say Sigma was possessed in the original continuity.

You can literally see the cutscene where he get possessed here.

In short, its outright canon that Sigma was possessed in the original continuity. Things are different in the reboot Maverick Hunter X, though, which is a separate canon.