The first man, Joseph Rosenbaum, chased Rittenhouse and attempted to take his firearm from him. If he would have succeeded he would have been armed and Rittenhouse would not have been.
If I have a weapon in my hand and you tried to disarm me and I had reason to believe if you acquired my firearm you would shoot me — I would be in fear for my life or health and shoot you to stop your attack.
If you are close to a person and have training it is not all that hard to take their weapon from them. Unfortunately if you screw up you may end up shot.
The situation can be compared to what happens if someone tries to disarm a police officer.
Cops have to make split-second decisions in situations that are tense, uncertain and rapidly evolving
www.police1.com
***snip***
Trying to disarm a peace officer is not only a felony offense in most jurisdictions, but it is a serious threat of being killed or seriously injured. You don’t have to wait until you are disarmed to take appropriate action to defend yourself. It is reasonable to assume that if they are bold enough to try and disarm you, they are likely intending to use that firearm against you. Using deadly force at that point would be a reasonable response.