In the UK, we have a routinely unarmed Police force. Gun crime is also fortunately only a very niche issue and we have specialist units that deal with armed offences. The justification is below:
I've always been of the view that one gun in a potentially violent situation leads to a lower potential for being shot. Both sides having guns merely raises the stakes and in hazardous situations, simply increases the chance of being shot from panicking the criminal. This notion of 'defending oneself' is exactly the reason why knife crime in South London has become a problem. One teenager carries a knife, the others start carrying to 'defend' and then all of a sudden you have a spate of stabbings. The same is true of guns.