What was the quickest way someone went from amazing to a huge red flag?

I had a guy friend with whom I was often chatting. We were talking about everything, including our crushes etc. I advised him how to talk to a girl he liked, I talked to him about guys who approached me. They were usually just light-hearted conversations, at it was good to have someone my age (we were in high school) to talk about such thing from the male perspective (and female perspective on my side). We were good friends. Even though he knew I was not interested in a romantic relationship with him, he suddenly started calling me “baby”, “honey” etc. It was the first flag. I told him I didn’t like it and made it clear how I see our friendship. Some time later he started saying very weird things to me, for example when I was travelling, he said “take care, I would kill myself if something happened to you”. It didn’t sit right with me and honestly made me super anxious. I was feeling awful most of the time and my friends and relatives started to notice that I am not okay. I eventually had to cut him off. He was just fine.