Am i too aggressive or just very dominant?

Hey, I am a 18 years old girl who people would describe as very ambitious and assertive. Today I went through a situation where I had to argue against a girl for my friend's sake,afterwards my friend told me that I wasn't loud and I did not swear,but I was very aggressive? The situation was this girl who I am very irritated by did something to my friend that my friend doesn't like and she used to do that to me too but I never had the chance to tell her about it. So when she did that to my friend 1. It felt like it was my time to share my feelings since no one was there,2. my friend is not the type to say no to people,she's too nice. My friend told me that it sounded very aggressive like " do that again and i will kill your entire family " type of aggression in a really calm voice. I feel very bad because I never realized that I could be this aggressive and also I've always believed that a little of aggression will help me with my future career. Is it bad to be aggressive in the way I've explained? Will it actually help pushing me forward? If not, how can I stop it? To be honest I still have no idea what the definition of Aggressiveness?????