Hello Danny, "Lo malo es cuando se hace vicio". Here vicio means " bad habit" The literal translation is : " The bad thing is when it becomes a bad habit". It means that the immorality of any action reveals its presence when it is repetitively done rising into a bad habit, because probably it turns then into an addiction that is hard to give up .

It means "The bad thing is if it becames a vice". Meaning that good things are good as long as they aren't a vice that you can't control any more. You'r wellcome...