The argument is that Didi made it easier for this alleged killer to get his victim into a vulnerable position (as a presumably backseat passenger in his car).

Didi allegedly did not respond to multiple negative comments about this guy's behavior, nor did they expose these comments to future passengers, which might have flagged him as a threat.

If not for Didi, or if they had been more safety-minded as a company, then this guy would not have had the opportunity to rape and murder this woman, allegedly. That's the argument.