You cannot kill someone for property. The idea of open carrying rifles is for personal protection and, IMO, intimidation. "Don't fuck with us. Don't fuck with this property." Neither things are illegal. Is it good judgement? Not for Rittenhouse. Should've never been there. But what he should've done and what was legal are two separate things. Simply put, open carry is not illegal in Wisconsin.
The hunting question is almost immaterial. It may have been the legislators' intent to make rifles only legal for hunting purposes, but that's not how they wrote the law. How they ultimately wrote the law made it legal for Rittenhouse to possess the rifle. So this is on a gun law sloppily written. If Rittenhouse had other types of guns, it would've been illegal. The law is weirdly clear on it. When the defense brought it up for the judge to dismiss, the prosecution didn't even fight it. They knew Rittenhouse didn't violate the gun law.
Rittenhouse isn't facing any weapons charges now. One of the ironies of this is, the friend could end up serving more time than him.