Ian Huntley is singled out because you would have to be totally insane to think that he could ever really be a woman and he is high enough profile that most people in the UK won't need to go and do a ton of research to know who he is
But you could have chosen anyone couldn't you? Vincent Tabak, Levi Bellfield, Rolf Harris, John Wuorboys. It's a hypothetical argument because Ian Huntley is not saying that he is a woman either.
The question then becomes 'can an evil criminal man change gender'? Well under current law, yes he can, despite being an evil criminal. There is nothing to prevent IH seeking a GRC if he can convince psychiatrists that he has gender dysphoria. It will become much easier in the future, but it is still possible now.
If the question in fact is 'can a man ever become a woman', meaning can biological and chromosomal changes be achieved, then the answer is no. Then it makes zero difference whether the man in question is good or bad because it is just not possible.
I think everyone agrees that IH is not a woman though.