Yes the police should ideally ensure that the streets are safe.
The reality is different though, I'm not saying it's acceptable or right, just the reality.
I'm a guy and an ex-bobby and there are areas/places that I would not walk alone at night. Not because I'm scared but because I know the reality of the risk in certain places at certain times and my choice it to remove myself from it, unless I'm being paid and I've got the lawful means to defend myself and get help quickly.
People go to into the hills, ignorant of the risks and they get into bother. Most Mountain Rescue call-outs are to incidents that were entirely avoidable if people had been aware of the risk and made informed decisions.
There are mad, bad people out there, there always will be and the police can't be everywhere. We all need to consider the risk in everything we do. If we are uncomfortable with the degree of risk and can't mitigate it sufficiently then the wise thing to do is to not go ahead and find a better plan.