Oh no he didn't! Cloud just...SPOILER ALERT! (really?-_-)
killed Aerith.. well according to this awesome theory by Matthew, Sephiroth isn't the real suspect, he just paralyzed Aerith, maybe so that she can't assist Cloud during their fight, but this emo bastard Cloud quickly assumed she's dead and tried to be the good guy by giving her a "proper" burial by sending her underwater. Check out the full video game theory below.
Sephiroth: "What? I can't even... Why?"
Check out other cool gaming theories by Matthew.