Hiking the Appalachian Trail is pretty safe but just like anything else the Appalachian Trail can be a dangerous hike especially if you're hiking it alone.
When you hike the Appalachian Trail it's best to hike the Appalachian Trail along with someone else so that in the event you need help you won't be alone.
Some possible dangers of hiking the Appalachian Trail alone are getting hopelessly lost or falling ill, wild animals attacking you and bad weather or dangerous weather conditions etc.
Usually though most people are safe and make the journey safely through the Appalachian Trail.
There's also the risk of someone harming you along the Appalachian Trail but most people are nice and willing to help one another out there.
Still no place including the Appalachian Trail is immune to any dangers or crime.
The most danger most people really would face when hiking the Appalachian Trail though is getting bit by a tick.