I'm worried my brother is gonna be a school shooter.
What makes you think your brother did that?
I heard he argued with a couple of guys at school and said he wants to kill them.
Threatening to kill people and to shoot up a school is a serious thing. Even though he is your brother, you should absolutely contact an authority figure and tell them about this. If you don't people could die if your brother decides to actually go through with it.
I don't want to get my brother in trouble or make him upset.
You need to do this. You could save lives and get your brother help before he destroy his life and others.