Police union was critical of Giuliani when he was Mayor--and he was probably the most pro-police Mayor of modern times:

http://gothamist.com/2014/12/22/police_unions_nypd.php

One of the differences between Giuliani and DiBlasio is that Giuliani was (is) a right-leaning Republican, but he kept his ideology in check when he was Mayor of a mostly Democratic city. He was pragmatic and focused. DiBlasio is a prisoner of his liberal ideology and constituency. That's why he made stupid comments--and is paying for them.