Chapter sounds crazy AF, but I can totally see 2 kind of scenarios:
1. scenario where Yuji is pulling Sukuna back into him with some kind of binding, thaught by Gojo. And we get a small Gojo-Smiley with thumb up, because he already planed this situation. Maybe it is also part of the contract sukuna made with yuji.
2. Sukuna took over Megumi for a good intention. He knows Megumi is not able to fight his Sister and he is also not able to watch Yuji doing it without interfering. So he took the easy route, take over Megumi to kill the imposter in his sisters body. And after he can claim he let Megumi watch and just wanted to feel the pain of Megumi.
so far my theories. I still don’t think Sukuna is the absolute bad guy.