The Indians in that massacre were not US Citizens AFAIK, and therefore it was killing enemy fighters. Carpet bombing Dresden in WWII and nuking Hiroshima & Nagasaki also had much bigger kill counts. Not the same thing as murder of American civilians. Comparing Orlando with Wounded Knee is apples and oranges.
Wounded Knee was a mass shooting of Americans, unless you don’t think Lakota Sioux Native Americans in 1890 were Americans.