No Sephiroth does not kill Kadaj its like this: Kadaj, Yazoo and Loz want to gather the remnants of Jenova, which I dont know why, Rufus possesses. Since they possess the different parts of Sephiroths consciousness they want to do the reunion with Jenova to become the one and only warrior, Sephiroth. When Kadaj finally receives the parts from Rufus, Cloud had been healed from his Geostigma and chases the three. He beats up Loz and Yazoo but Kadaj can get away. Cloud traces Kadaj, fights and beats him but then Kadaj fuses together with Jenovas remnants and becomes Sephiroth. Although normaly for the reunion all three parts of Sephiroths consciousness are needed, in this case Kadaj alone suffices(since he represents Sephiroths cruelty). Then Sephiroth beats up Cloud but then Cloud can somehow collect energy again and defeat Sephiroth. Sephiroth transforms back into Kadaj and Kadaj dies. Yazoo shoots Cloud and Cloud sinks to the ground but in the little pond in Aeriths church he gets healed again as do the other children(which suffer Geostigma).
Hmm…I didn't get this impression at all from the movie.
…they want to do the reunion with Jenova to become the one and only warrior, Sephiroth.
Yazoo, Loz and Kadaj never state that they want to become Sephiroth, only that they want to be reunited with JENOVA and spread her virus. In fact, in a conversation between Kadaj and Rufus, Kadaj claims that he dislikes Sephiroth because he believes that JENOVA may love Sephiroth more then him.
Since they possess the different parts of Sephiroths consciousness they want to do the reunion with Jenova to become the one and only warrior, Sephiroth.
…
…but then Kadaj fuses together with Jenovas remnants and becomes Sephiroth. Although normaly for the reunion all three parts of Sephiroths consciousness are needed, in this case Kadaj alone suffices(since he represents Sephiroths cruelty).
I got the impression that when Kadaj turns into Sephiroth, it was more of a possession then Kadaj actually turning into Sephiroth. Sephiroth needed a physical vessel so he manipulated the last Remnants of JENOVA so he may get at least one of them to take over their body for a Second Coming.
I don't believe that "hypothesis" that they, Kadaj, Loz and Yazoo, are Sephiroth, split in 3 personalities (as there is no evidence as such and is merely pure speculation). However, there is evidence that they are being controlled or manipulated but either Sephiroth or JENOVA (I'd say the later) just like Cloud was in the original game. That being said, they are most likely survivors of those hooded "Clones" (the clones weren't really Clones but more like test subjects subjected through several Surgeries and injections to try to create another Sephiroth…in a sense but they weren't really clones.) who eventually regained some form of their Identity but were puppets to JENOVA, just like Sephiroth was.
Kadaj, Yazoo and Loz want to gather the remnants of Jenova, which I dont know why, Rufus possesses.
…
When Kadaj finally receives the parts from Rufus,…
The "Parts" is actually JENOVA's Head (and juices). And Cloud is healed of his Geostigma in the Church, after he defeats Loz and Yazoo, not before that chase scene.
Oh and Kadaj, Yazoo and Loz ARE the Remnants by the way (though they do believe that anyone with Geostigma are also Remnants so they are looking for the Remnants).
Yazoo shoots Cloud and Cloud sinks to the ground but in the little pond in Aeriths church he gets healed again as do the other children(which suffer Geostigma).
I don't know where you got "and Cloud sinks to the ground" because that never happened.
Who is controlling who? Is Sephiroth controlling the Jenova inside him or is Jenova controlling Sephiroth from the inside out?
I have a entirely different perspective which is that Sephiroth is Jenova. lol
Jenova was basically a humanesque virus whose sole purpose was to spread herself across the planet and take over. However, she had no feelings of happiness, sorrow, or guilt, just the instinct to take over. Sephiroth, after learning about his mother being an alien began to act this way as well, but kept his feelings (for example: his arrogance when fighting Cloud, and wanting to know what Cloud treasured most so he could take it from was desire). In a way I think Sephiroth became the next evolutionary step of Jenova... if that makes sense. He wants to destroy life on the planet so he can use it to travel to a new planet and be it's ruler.
I would say that Sephiroth, at first, was merely brainwashed or possessed by the JENOVA cells within him after realizing the "False Truth" about himself and his mother JENOVA (and I say false truth because of Hojo fabricating the books in there after destroying the originals as he knew that day, this would happen. And to ensure that it did happen, he sabotages the Nibelheim MAKO reactor to produce Monsters…and I know this because Sephiroth came to that conclusion before he lost his mind.)
Before the Nibelheim incident, Sephiroth had a completely different personality. I'd like to imagine as that Incident was taking place, in Sephiroth's mind, he was battling it out with the JENOVA within him or a Dark Doppleganger, representing his future self. As of course, the Original Sephiroth lost and, as I'd like to believe, died, where JENOVA fully took over with Sephiroth's personality somewhat intact when Sephiroth's body lands in the Life-stream.
Now you may ask "why does Sephiroth when he's JENOVA call JENOVA mom". Well, it may have a biblical reason to it. Sephiroth is JENOVA but Sephiroth is the son of JENOVA, much like Jesus is God but Jesus is the son of God.
Sephiroth, before the incident, Even though he believes JENOVA to be his mother, I consider this Sephiroth to be the Son of Hojo and Lucrecia while the Corrupt Sephorth is only the son of JENOVA. I don't consider then as the same person. Only the body and voice is the same. Even the strength differs.
And why JENOVA's Sephiroth want to destroy all life? JENOVA wants revenge!