Either the killing was pre-planned in which case Momeni would have found or created an opportunity to carry it out regardless, or it was a fit of rage situation in which case you would need to have multiple police per city block at all times for there to be a reasonable likelihood that an officer could intervene in time to prevent incidents like this. Random crime you can police your way out of to some degree. This? No.

Of course this is all assuming the arrested individual did it.