Is it normal to follow my cousin?

I had a problem in my family. Nothing to do with me really, it was my aunts daughter. She dated with a man who is 5 year older then him, normaly it wouldn't be a problem, but since she was 17 years old and man was 22 years old. I was concerned and watched(or stalked?) her for a while and I saw the look at the mans eyes. He was tricking my cousin(btw she was and still my dearest cousin and friend in my life)after a talk at the cafe or bar don't recall... He and she get in to a car. They rode off fast, but I found a taxi and followed them. But I couldn't(we couldn't) caught up with them. but after a few blokes later. I saw the same car near a house, I looked from the window but didn't saw anything. I didn't tried to climb the top, since I know I would fall down so easily. After a few minutes I got the courage of knocking the door. (I followed her because I was worried, she was inmature and I didn't know what was that mans intentions.) That man opened the door, and at that time I couldn't stop myself and attacked him. He got unconscius after a few seconds, that was the time I saw the look at my cousins eyes, she was scared and then she told me that she hates me and she ran off the house. Since then she didn't talked me, only thing she said me is "you should have trusted me, after the years we spend together I thought you knowed me." These words echoes on my head all the time, so I wonder, if that is normal to follow her like this. and what should I do to regain her trust?