Kyle Rittenhouse -- who shot and killed Joseph Rosenbaum and Anthony Huber during a 2020 Black Lives Matter protest in Kenosha, Wisconsin 6 years ago -- wound up in the hospital this week ... after he says he was bitten by a spider.

He posted some pics from a hospital bed on Wednesday, writing ... "The communists couldn't take me out and i'll be damned if I let a brown recluse take me out."

The selfie shows him hooked up to some monitors, while the other image shows circles drawn around a red splotch on his leg ... where he was allegedly bitten. Brown recluse spiders are no joke -- the bite can cause serious side effects ... sometimes taking months to heal.

He posted another pic on Thursday of himself aiming a rifle out a window, writing in the caption ... "The spider, like the commies, also thought it was a good idea to come after me while I was armed. He did not survive."

Rittenhouse's post was flooded with right-wing well-wishers ... including Kentucky Senator Rand Paul.

Rittenhouse was previously charged with intentional homicide, reckless endangerment and other charges for the AR-15 shooting in 2020 -- a third person was also struck, but survived -- but he was acquitted in 2021.