As far as I know, Sephiroth had at least 2 chances to kill Aeris by manipulating Cloud.

Scene 1: Temple of Ancients
Cloud attacked Aeris after giving Black Materia to Sephiroth.

Scene 2: Forgotten Capital/Forgotten City (The end of Disc 1)
Cloud raised his sword (huh?) just before Aeris died stabbed by Sephiroth.

Perhaps, Cloud didn’t love Aeris, that’s why he could even be controlled by Sephiroth to kill her if possible. Then again, how could he be stopped by his comrades at that time? I bet that he somehow loves…

Discuss: What if Aeris was killed by Cloud but not Sephiroth? There are a number of possibilities, who knows?