Should I be worried my oldest son is going to have problems

So my oldest son is 4 years old and he is getting really mean to his sibling. My youngest bugs him and does get annoying but I feel like it is so wrong. Today he hit him In the head with a car and he is just getting more violent with him. My oldest son called me a stupid dumbass the other night and I yes I know they hear all the bad words you say but we are really careful. I am worried he’s going to be a serial killer or something. Everyone reassured me and says it’s a brotherly thing but it doesn’t seem right at all.. we are a normal family no violence in our household we are close and spend all our time together. The thing is also my husband is in denial and always finds a reason why he is innocent. He’s like o he must of tripped but I physically watched him hit him. Please don’t judge me I just don’t know what to do