Although not an overt publicity seeker, bass player, lyricist and principal composer Phil Lynott realized early on in his career that gimmicks and outrageous shenanigans generate considerably more publicity than saintly behavior. "Killer On The Loose" was obviously written with the intention of courting controversy, and it succeeded.
The lady-killer on the loose in this song was Jack The Ripper, which would have been bad enough, but when it was released on the Vertigo label in September 1979, backed by the arguably even more provocative "Don't Play Around," there was a real Ripper on the loose in the UK.
Although his reign of terror started some years before, the first documented murder by the Yorkshire Ripper was of prostitute Wilma McCann on October 30, 1975, but unlike the original Jack The Ripper, who is generally credited with five victims, all of them prostitutes, Peter William Sutcliffe murdered 12 women before he was caught, and his victims were not exclusively whores; in 1979 he murdered two decent young women, which aroused extreme public anger especially in Leeds, which was at that time a stronghold of feminists.
All of this was obviously known to Lynott, whose band was even bigger in the UK than in Ireland at the time. "Killer On The Loose" was re-released the following year as the B-side of "Hey You"; the band also made a suitably tasteless video to promote the song.
Peter Sutcliffe was arrested by chance in January 1981, in the company of a Sheffield prostitute. Convicted of thirteen murders, he was subsequently gaoled for life.
Suggestion credit: Alexander Baron - London, England
Zebedyzak from UkPhil Lynott apparently denied that he wrote this song with the intention of courting controversy, but.... maybe he did after all. Whether he intended it to be controversial or not, the song was certainly not released in 1979. I don't know what freak copy you have in your own collection that says 1979 on the label, but it was in fact released in September 1980, just a few weeks before the album "Chinatown" on which it appears.
As for "Don't Play Around", I hadn't thought about that one being especially provocative. It was not included on the LP.