Speaking as a mother in a similar situation who didn't, I would suggest cracking down quite hard on this.
At the time, I responded to it as if it was a fight between two children - I think because they were both 'my babies', and I could see in some ways DS1 was less mature than DS2. However, with hindsight I can see that from DS2's point of view, this massive adult attacked him, and his parent did nothing about it. :(
To my shame, it wasn't until DS1 threatened me that I realised quite how frightening that was.
I would suggest you tell your DS1, in no uncertain terms, that you absolutely will not tolerate physical violence in your family. I'd emphasise your total disapproval and disgust at people who hurt others smaller and weaker than themselves, and at those who take their stress and anger out on other people.
I'd also probably tell him that what he did to his little brother is assault, and that if he had done it to anyone else, he would be arrested and would now have a criminal record. I'd tell him that you believe his little brother deserves your protection - quite as much as a stranger or even more - and that you have been debating whether to call the police this time. I'd tell him you haven't, this time, but you will next time. And mean it.
Tell him you'd do this because you love him and you cannot allow him to grow up hurting people, or using anger as a first resort, or ending up in prison.
Last but definitely not least, I'd tell him he must now learn some other ways of handling his anger and frustration, and that you will help him find out about that. Tell him this isn't optional: he needs other ways of dealing with anger, and he needs them now. He is no longer a child, he needs to have adult strategies... Then I'd do at least one of three things:
- Send him away to google/research 'anger management'. Tell him he has until tonight (or whenever) to come back to you with at least 5 things he could do instead of hitting his brother next time he's angry.
- Research this with him.
- Find anger management support for him.
Do it now. Because if you leave it until he's any older, or used to getting away with it and seeing you as helpless and powerless, you probably won't be able to stop him.
Good luck.