The idea of 'crowdsource murder' is really disturbing. There may be some fictional stories or very rare and unreported cases that might seem like it, but no widespread or known true story. In our society, we have laws and ethics to prevent such abhorrent things. We promote cooperation for positive things like solving crimes through legitimate means like police investigations, community watch, and forensic science, not for any form of murder.
If a crowdsource murder were to occur, there would be chaos. The legal system would be severely challenged as it's a completely abnormal and illegal situation. Innocent people would live in fear not knowing who could be part of such a heinous act.
It's not a true story. Usually, such titles are chosen for their intrigue and don't imply a factual basis. Authors often come up with fictional plots to entertain and engage readers.