"I see," said Professor McGonagall, fixing Harry with her beady eyes. "Then you should know, Potter, that Sybill Trelawney has predicted the death of one student a year since she arrived at this school. None of them has died yet. Seeing death omens is her favorite way of greeting a new class. If it were not for the fact that I never speak ill of my colleague --
Professor McGonagall broke off, ...
I don't quite understand the sentence above in bold. What does it try to convey? Particularly the part: If it were not for the fact
-- From Harry Potter.