Trannie has the connotation of a male hooker dressed as a female, if I'm not mistaken. I'm pretty sure the word all together has a different intended meaning than transexual/transgender. I guess if you put it in context, calling a transgender/transexual person a trannie would be the social equivalent of calling a female a "whore". And like the term "whore" you don't just want to throw it around willy-nilly, probably want to save that one for a rainy day. Really cuts deep.