Idiom/metaphor for a certain kind of robbing

I think the most natural English phrase that would fit your context would be "Turn to a life of crime, deepen myself...."

It doesn't necessarily mean that the subject would fund their new lifestyle through violence, but that they would be living and financing their life outside the law.

You could always qualify it with "Turn to a life of violent crime" if the violent aspect is important.

It's not an idiom as such, but then, as Mari-Lou pointed out, neither is "Hit people in the head"!

Robbing itself is the act of thieving using coercion/force. So a Robber would be a proper word to.

You can use some alternatives like Marauder/Plunderer

Maruad

"go about in search of things to steal or people to attack."

Plunder

"steal goods from (a place or person), typically using force"