Just because you're shooting the shit with friends and mention something along the lines of "somebody ought to do something" does not make you forfeit your right of self defense. Rioters were burning down the city where he worked and his family lived and looting neighborhood stores. Every idle remark said in the wake of that isn't proof positive of intent. Even BLM activists that open carry or concealed carry w/ permit at the riot don't forfeit their rights just because in the weeks or months prior they wished white supremacist cops would die.
It's really torturing the law to claim otherwise.
For the record, the video I'm remembering doesn't show Rittenhouse and nobody in the video makes the claim that it is Rittenhouse speaking. I'm guessing its just the word of the reporter that found the video that it's Rittenhouse. That's pretty sketchy still, even if it doesn't have any ultimate bearing on the justifiable homicide.