What is the ruling on working as a lawyer in criminal law and defending criminals for a fee in a European country?

The default ruling is that it is impermissible to work in man-made courts that do not rule by what Allah has revealed, whether the work is in the judiciary or in law, due to the Almighty’s saying: "And whoever does not judge by what Allah has revealed - then it is those who are the disbelievers," and His saying: "Have you not seen those who claim to have believed in what was revealed to you and what was revealed before you? They wish to refer for judgment to Taghut, while they were commanded to reject it; and Satan wishes to lead them far astray. And when it is said to them, 'Come to what Allah has revealed and to the Messenger,' you see the hypocrites turning away from you in aversion."

An exception to this is necessity, which requires the presence of Muslims in those courts to remove injustice and defend their rights, or to achieve their interests and ward off corruption from them, especially in countries where there are no Sharia courts. A Muslim lawyer is not permitted to defend criminals, due to the Almighty’s saying: "And do not argue on behalf of those who deceive themselves. Indeed, Allah does not like whoever is a betrayer, an arch-sinner." If the objective of the work is to support the oppressed and demonstrate the justice of Islam, then it is an honorable work for which its doer will be rewarded. However, if the purpose is employment and salary, then it is not permissible according to Sharia, and a Muslim must seek a lawful means of livelihood that spares them from the unlawful.