You forgot the transexual serial killer option.
It's hard to say. It would require an incredible level of naivete on the part of a teenager, especially a black teenager*, walking around the streets late at night and being watched not to realize he may be being watched to make sure he's not up to no good. But without know exactly how it went down it's very, very hard to say what went through TM's mind that night, and it's entirely possible he thought it was something more sinister.
Like Giz, I have a hunch in that regard, but it could well be wrong.
*Edit: So that this is not misunderstood as some type of racist comment, I'm saying that black people are used to being profiled and suspected of something whether by police, citizens etc., as racial bias is still so deeply engrained in our society.