Yes, Bester’s a villain – but Sinclair has no way of knowing that

I don't think so. Sinclair may not know the extent of Besters villainy, but he will have realised quite soon that he was bad news. Bester does not make any secret of his attitudes and if he tries to communicate telepathically to save time, it is unlikely that he is any more polite in doing so.
Also, these is some suggestion that the PsiCops have a reputation,something that would hev informed Sinclair's reaction, even if it had merely been based on prejudice.