I understand if they were hunted for meat or fur but I’ve heard of “hunters” just shooting them from the tops of trains and leaving the bodies to rot.
Were they somehow a nuisance or just considered good sport to kill?
In the hunts you describe, the buffalo were intentionally killed as part of the American genocide of native tribes. You can read more about this in this series of threads from u/snapshot52:
And
Hi there. I've written about this subject two previous times. You can find those here, where I explain one factor as to why the buffalo wasn't seen as a food source for Euro-Americans, and here, where I explain that the U.S. Army actually developed a policy to purposefully exterminate the buffalo herds in order to starve out Indian Tribes on the Plains, which constitutes an act of genocide.