In the name of Allah
In regard to your question
What happens if you kill someone during Hajj?
In such situation as you said “if you kill someone during Hajj?” it seems invalidates the hajj. Of course in regard to this matter as you said: if someone trampled over someone with the intention to get passed them in order to complete their Hajj," I didn’t find this item as a Mubtalat of the Haj in exactly the same item.
On the other hand, in the mentioned situation (as you said trampling on other Hojaj), occasionally some people hurry up in order to do their haj faster, then unintentionally it could be happened because of the population. But I assume it can be impermissible and even invalidates the Haj if someone do it intentionally.
But in the above-mentioned example (this year, Haj), we’d better not to accuse the Hojaj directly, since as much as I find out, it is happened due to the lack of a good management in that position by the persons in charge that apparently some responsible closed a way of the Hojaj, then Hojaj went in an awful situation unintentionally, afterwards… Hence it doesn’t seem to invalidate the haj of the hojaj who unintentionally encountered with this situation... Of course generally (except this year accident) it can be impermissible for hojaj to annoy other hojaj by trampling as you mentioned (intentionally).
But concerning the initial part of your question as you mentioned “Approximately 717 people died in this event.” Seemingly there are different statistics about it. Actually as much as I searched, it is said between 769 to more than 4100 people have been killed (according to different sources).
It could be impermissible if Hojaj trample on other Hojaj intentionally. (Although majority of them or even all of them will observe the right of other Hojaj.)
God Bless You