N.K.
Sounds to me like he is having trouble coping with and expressing his feelings and emotions, so it comes out as anger and frustration, in the form of dramatic, negative, concerning words. As to how he is towards his baby brother, perhaps it's a result of not knowing how to behave around a younger child. He is only 7, and no one is born knowing how to handle babies. He may not mean to tackle and be rough to his brother, he may be trying to show him affection and instead he is getting punished for showing his brother love, and he gets angry and frustrated and says such things. I think you all, as a family, should talk about his behaviors, how to properly cope with anger and frustration, and how to play and be attentive to his little brother in a safe way that won't harm him, or cause either of his parents to yell at him or ground him.