1) Murders are not evenly distributed in most (all?) cities. In Chicago, for example, violence is heavily concentrated in certain areas. That means it's not a generalized problem yet this app doesn't address that clustering in a meaningful way.
2) Murders are not the crime that most people will feel comfortable intervening in – and, arguably, not one that is most high-value to report in the moment – unless you're focusing on armed, trained responders – who are likely going to be Police (unless we're getting into militia here and that's a whole different discussion). Instead, less violent offenses seem more manageable as a starting point. What if a purse snatching was reported?
To be honest, I'm still not sure what value the app provides – generally speaking, this looks to encourage mob justice which is fraught with risk of mistaken identity and violations of individual rights.
For the creators – what, specifically, are you looking for members of the public to do?
This strikes me as another one of the AWARENESS!! "solutions" – assuming that awareness is the problem – honestly, especially in crime solving, I don't think that's the issue. Awareness of identifiable precursors to crime? That'd be really useful (what they are, I have no clue). Awareness of active shooter situations? Also useful – but much more rare and less about fixing an more about avoiding for the general user...