yes with less cash there will be lesser cash robberies and death, but eventually this criminals will adpat, and may target that are openly involved in bitcoin transactions like maybe a store that says it accepts bitcoin
People will still mug others. They would still your phone because they would know that you had a Bitcoin wallet installed on your smartphone. Fake stores are already a thing, mostly on the Internet and many people got scammed in a such way.
The biggest advantage is that we won't need ATMs and banks where you can deposit and withdraw money. Less risk for people who work there.
There are various kinds of protection which can prevent from stealing data from a smartphone. At least, even now most smartphones have passwords and fingerpring protection which are pretty difficult to hack. Most likely, street robberies will be replaced with digital ones and hackers will be attacking some intermediary services such as MyEtherWallet.