The missile was definitely part of a barrage of missiles fired by Palestinian Islamic Jihad. That part is not in question and has been confirmed by the Pentagon. It also appears that someone was using the hospital as a place to store explosive weapons which were set off by the falling Palestinian Islamic Jihad missile. That said, there is a part of the narrative that is being ignored by both sides as they tell the story and Israel may not be entirely blameless.
Most of the footage is conveniently zoomed to crop out the top of the frame. You see the flash of the rocket motors as the missiles were launched and you see the explosion at the hospital, but nearly all of the footage doesn't show the small pop of light in the air just before the explosion at the hospital. That could be the rocket malfunctioning or it could be the rocket being intercepted by iron dome. I'm thinking Palestinian Islamic Jihad fired off these rockets, The iron dome defense system intercepted at least one of them while still over Gaza, and the remnants of the intercepted rocket fell on a hospital Hamas was using as an ammo dump.