How much sibling fighting is normal? Kids are 3.5 and 1.5

So my son is 3.5, will be 4 in August. His younger brother is 1.5, closer to 2, will be 2 in July. My oldest son is constantly messing with his younger brother. I mean randomly going over to him unprovoked to hit him, kick him, step on him, pull on him, and recently he’s been shaking him. We are NOT a violent household and he doesn’t see anything like that on TV, so I don’t know where he’s getting it. He says he “loves brother” and will also do sweet things like kiss and hug him and help feed him but for some reason he can’t stop randomly harassing him. He hurts him at least twice a day even with such strict monitoring. I’m always around. Just a while ago we were all sitting on the couch together after coming in from outside. I put Peppa Pig on. Randomly my eldest came over to my youngest and punched his arm as hard as he could. Like... why?! I got so mad I stuck him in his room with the door closed for ten minutes while he screamed begging me to let him out. But I’m so damn frustrated with this. Why does he spontaneously decide it’s time to hurt his brother? I do not get it!