Hello Emma, In my personal opinion, both are incorrect though even native English speakers would say the same thing. I think what you really mean to say is; "They are using their belief as a reason to be criminal and violent." Or perhaps; "They are using their belief as a crutch for criminal and violent behaviour." However, if you want to use mean in the sentence, you would say; "They are using their belief as a means to be criminal and violent."

Hi Emma: "They are using their belief as a way to be criminal and violent." Is is possible to replace "way" by " mean" ? Very close Emma. Here, the correct use would be with an -s ending, as in MEANS. So the sentence would read like this. "They are using their belief as a MEANS to be criminal and violent." Also Emma, let's look at some variations for communicating this idea. "They are using their belief as a way to be criminal and violent." "They are using their beliefs to justify criminal behavior and violent behavior." "The are using their way of life as a means to survival." "Criminality and violence are the means by which these people live." "Their violent ways and means draws them more deeply into the criminal life."

Is this what you mean? "They are using their religion as a justification for their criminal and violent behaviour."

No.