Yes. It could be considered a threat. Here's an anecdote from one of my personal experiences:
A few years back, I had an argument with my wife, and it got pretty heated, so I decided to go for a walk. It was around 8 or 8:30 in the evening.
My walk took me to the Santa Monica pier, and for those unfamiliar with the area, there are houses lined up along the shore (a good distance from the ocean, but there is a walkway there).
So, I'm walking along the sidewalk towards that area (it basically looks like a "T" junction, I was walking to the top part of the "T", where I'd be parallel to the ocean), when I spot a guy leaning against a building. He's staring at me something fierce, so I do what I always do when someone makes eye contact with me, and I give a slight nod of my head in acknowledgement. I reach the end of the "T" junction, and glance with my peripheral vision and see the guy following behind me. I didn't think much of it, however. I round the corner and am now walking parallel to the beach, with the houses to my right.
I started feeling a little strange, so I glanced over my shoulder, and there was the guy, standing at the start of the junction, watching me. I keep walking, and it's darker than I thought it'd be along this street. Usually there's people milling about, being on the beach and all, but that night, it was fucking dead. Nobody there. After walking further, I look back behind me again, and the same guy is walking towards me, with a brisk pace.
I didn't turn and run, but I walked as fast as I could to the next "T" junction and turned back onto the main street, where, thankfully, more people and open stores were available.
I personally felt uncomfortable and threatened by the actions of that guy, even though, it's very, very possible, that he just happened to want to go that direction at the same time I did.
His previous behavior (the hostile looking stare that he didn't break), and him following behind me led me to believe that, if he caught up with me, he may hurt me. I was, physically, preparing myself to throw down if I had to.
I don't think it was unreasonable for Trayvon to be afraid of Zimmerman following him. And I think the dismissal of someone being uncomfortable or threatened because someone is following them is a little absurd.
Now, if Trayvon was Katrina Martin, and she was on the phone with her friend and saying that a strange man in a car was following her, then following her on foot, and she subsequently ended up shot dead by that very same guy, people would be calling for justice even more fervently. Why shouldn't a guy, and most importantly, a teenage boy, be afraid or uncomfortable in the same situation?
I may be a man, but I'm not afraid to admit when I'm scared shitless or uncomfortable. Being followed by someone in a vehicle is suspicious. When that person exits their vehicle and continues to follow you? That's hostile. At least in my opinion.
