A reader claimed the Minnesota Massacre of whites was worse than the Bear River Massacre of Indians. Here's how I responded:
A few problems with designating the Minnesota Massacre--aka the Sioux Uprising of 1862--as the worst, Anonymous:
1) The article I cited says almost 500 Shoshone Indians were killed at Bear River. Wikipedia says 200-400.
Meanwhile, the link you provided says more than 450 civilians were killed at the Lower Sioux Agency near Redwood Ferry. Another source says more than 400.
In short, the numbers are in the same ballpark. We can't say definitively that one massacre was worse.
2) The Sioux under Little Crow held a war council and officially declared war on Minnesota. Their first attack was on a government agency, which arguably was a legitimate target. In other words, the civilians were killed during a military attack; they weren't massacred in their sleep or after they'd offered to surrender.
For a comparable situation, consider the Japanese attack on Pearl Harbor. We don't call that a massacre because a) Japan was at war with US allies and b) Pearl Harbor was a legitimate military target (to the Japanese). If Japan had attacked a civilian population with no military resources, as we did at Hiroshima, then we might call it a massacre.
3) The main Indian attacks occurred over a week--from Aug. 18 to Aug. 25, 1862. A massacre arguably has to occur in one fell swoop--in a single day or so. If the defenders know someone is after them and they have time to prepare, they're arguably participants (even if unwilling) in a war. In that case, they can't be "massacred"--unless, again, the attackers kill them when they're not fighting.
So the first Sioux attack on the Lower Agency August 18, and the civilians killed that day, may qualify as a massacre. But any attacks after that arguably were battles, not massacres.
Sources for further reading: