'Mad Cold-Blooded Murder' (1981) aka Yuan yin. – Tom-Bong Law.

There's a bloodthirsty psycho killer loose on the Hong Kong's seamier backstreets, and steel-thewed cops Bruce Le & Carter Wong undertake the increasingly dangerous task of bringing this maniac to justice! A cheap and cheerful chopsocky romp, Mad Cold-Blooded Murder remains an enjoyably brisk, fight-heavy 80s martial arts spectacular! Acrobatically enlivened by some of Kung Fu cinema's ablest practitioners: Bruce Le, Carter Wong, Chen hu-min, and bone-crushing bad guy behemoth, Bolo Yeung! Alongside some righteous rucks, this lively admixture of sleazy, backalley slasher motifs, boisterous Kung Fu shenanigans, and bangin' synths, has enough sinewy, shin splintering savagery to appease most old school 'fu freaks!!!! A deadly detecting duo, armed with fatally flying fists of ferrous metal, and cranium crushing kicks, Kung Fu cops Carter Wong and partner Bruce Le still pack an almighty wallop!!!! There's one nastily prolonged strangulation scene that suggests to me the copy I watched might be edited, since some of the kills are largely off-screen?

“That maniac's killed 12 whores!!!!!!”