J.R. Smith says he's disappointed in himself for opening a can of whoop ass on an alleged vandal ... saying it was a "random act of stupidness" on both sides.
TMZ Sports posted the video of the incident ... where Smith is seen unloading a bunch of violent kicks on a man who allegedly smashed his car window during the protests in L.A. this weekend.
Smith joined the "Pat McAfee Show" on Monday to give more insight into the situation ... and the NBAer admits he wishes the incident never happened in the first place.