Here's the reason why Rittenhouse was not guilty and Travis McMichael is guilty in the smallest nutshell that I can place it:
Both Rosenbaum and Arbery grabbed for the gun before being shot and killed and both Rittenhouse and McMichael claimed self-defense.
Rittenhouse was armed; but, he was retreating from Rosenbaum before Rosenbaum cornered him and grabbed for Rittenhouse's gun before Rittenhouse shot and killed him.
Arbery was unarmed, chased, and cornered - "caught like a rat" was actually said in testimony - before he grabbed at McMichael's gun and McMichael shot him.
So, Arbery was basically an unarmed Rittenhouse while McMichael was an armed Rosenbaum in this situation.
Also, you can't place someone under a lawful citizens arrest if you have no direct knowledge of the alleged incident.
They didn't witness him steal anything, they just assumed he did. They didn't even talk to someone who saw him steal something. Their claim to a lawful citizens arrest goes out the window.
So in that way, they were far more like Rittenhouse's attackers than they were like Rittenhouse.
Chasing someone down with weapons and threats without the protection of the law behind them. They were the aggressors.