Police don't have a mandate to protect the public. This has gone before the supreme court, multiple times. Even if they are present while you're being attacked, they can just stand and watch if they think it's not safe for them to engage. (And practically speaking, they can't do it anyways, they're not omnipresent - if you live in a big city your response time for being murdered is probably over 5 minutes (where I am, it's 7+ minutes). Lot can go wrong in 5 minutes)

They do have a responsibility to protect people in their custody.