There's something seriously wrong with me

I know I'm a horrible piece of shit for saying this and feeling this way but I need advice on what to do or if I should just remove myself from this situation entirely. My boyfriend has a 3 year old and we have a baby together. I always find myself wanting to scream at, kick, punch, slap his kid and I constantly think about doing it. It makes my heart beat fast amd I get excited. It's sickening 😔 I would never ever ever even think about doing anything like that to my own son and I've never laid a hand on the 3 year old but I'm worried that I will. What's wrong with me?