I thought he was going to just stab her, since Ven said he wasn't going to fight Vanitas, so Vanitas was just giving Ven a reason to fight him.
Kill her, that's what I thought MX's plan was, so Terra would completely give into the dark so he could take over his body, and give Ven a reason to fight Vanitas to make the X-Blade. He was never going to use Aqua as a vessel.
Well it seemed more like he was going to shoot a beam because he positioned his keyblade the exact same way MX did before he shot the beam into Ven's heart. And in a way, it could kill Aqua. I mean, Ventus almost died until Sora came to heal him.
But yeah, MX did say to Vanitas "Take everything away from Ventus and destroy Aqua!" The only people he planned on using as a vessel was Ventus and Terra.
Would have been odd if MX ended up possessing Ventus like he planned to... I mean, can you imagine that?
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