Excessive force? That's hard to judge, defending yourself is way easier to judge, but to make someone incapable of hurting a beloved it's hard to.
People should try to put themselves in the situation, if your wife (or mother, or anything like that) calls you in panic saying a maniac invaded your house and it's trying to rape her I don't think anyone would be calmly and thinking straight, once he got there he hould undoubtly attack the guy to protect that person.
Anyway, it might have been already settled and he wanted to charge his anger anyway, then got to beat up the guy even unneed, but even so, try listening to someone you love crying to you saying it's about to be raped and try to be cool, that just isn't real, rarely someone would hold itself.
No, it's not hard to judge. When he's outside in the hallway, it's pretty easy to see that he's unable to hurt anyone, so it's absolutely excessive force.
Also, if you actually beat someone to death by inducing severe trauma to both head and body, you can't just have done that by accident while trying to knock him out or subdue him. You have to have been aggressively attacking him. I don't feel sorry for the husband, he should get charged. Simple as that. I don't care if he did it in a fit of rage because his wife had called and asked for help. He murdered the guy and used much much more force than neccesary to protect his wife from the situation.
Yeah, if it was solely an anger explosion it was way too much, but that doesn't change the fact that it is, for sure, hard to hold yourself and someone you value so much (dunno if that's the case) is in actual danger, for example if he failed at taking an opportunity to hit the guy enough and could lose control over the situation, he could have died, his wife could have died, his wife could be raped and survived but had her whole life completely destroyed, mentality and emotionally, which could lead to suicide, which could also lead the guy suicide by guilt, etc.
When you take an opportunity to protect someone in need, even more someone that may be your whole world, you won't be calm, you won't be likely to be able to judge anything besides the need to protect, because as said above it might just backfire if you mishandle it, you do what needed to protect it until there is no more danger, it might end with a subdue, sure that would be ideal, but that's hardly the case, specially for someone without any kind of combat experience.
I'm not talking about kicking the head of someone already lying betaen on the ground, or something like that, reality is much more complex than that, there are just several ways things could have gone and several layers of misjudgement trying to avoid the worse outcomes for what is to be protected.
Once you go out of your way to endanger someone you endanger yourself at the same time, just like when someone is pointing a gun to a victim head and the police has to shot down that person to save the one that it deliberatedly put in danger, it's not the case here, but the act of protecting the victim is still the same.
Well, the police there know way better than I do what happened there and how it was handled, but it's definitely hard to judge the needed force to stop such acts, it's not about being an accident or not, it's about how much it's needed to actually protect.