Diesel, everybody has the right to choose their profession.
Many countries do not have any restrictions as of being a " Mercenary " ( we can debate what the term mercenary is ).
Should I choose to be a mercenary, there is no law in my country that will restrict me.
So if the guy wants to be a mercenary, it is his call.
I have never been one, just worked with bunch of them in the past.
Legionairs are mercenaries, I see nothing wrong with that or nothing the guys served in the legion can not be proud of.