"A lecture about the darkness? From you? Don't make me laugh." Cloud's disdain at the hypocrisy is real, but it's also covering a bit for how unsettled he is. He didn't even know he was capable of opening portals.

He has no idea what new mind game Sephiroth is playing with him now, but he can figure that out later. Right now, there are more pressing issues.

Like whatever is roaring at the end of this descent.

Cloud flips his sword around in his hands and shifts so that if whatever's down there happens to be standing just beneath the churning inkblot that is this portal, it's likely to get a very large and unpleasant sword-shaped wake-up call.