The painting is Friend or Foe by Robert Griffing. While he’s not native american, I can’t find anything to suggest his paintings were not made with a consideration for real history and an appreciation of the culture he’s depicting. His paintings are featured in many native american-run museums.
Plenty of pro-Palestine protesters have targeted allies (or at least uninvolved third parties) in real life, ie attacking pride parades, so unfortunately I don’t see why being on the internet would make things any different. Some people are just desperate to be heard and habitually pick the worst possible way to convince others to listen. Not ruling out intentional shit-disturbers but it’s never only that.