He made me feel much better, because it made my very bad score look quite respectable. That's the kind of thing Paul is talking about here. Paul doesn't even attempt to give a counter argument to such a spurious thought. It is just self-evidently stupid. You cannot do evil so that good may result.
Evil is evil, period.
And they are deliberately misrepresenting the gospel by claiming that Paul is promoting sin because he says you can't get to heaven by doing works of the law. Incidentally, we have a clear indication from scripture here that the end does not justify the means. That's not the main point of Paul's argument, but it is a distinct principle that we must use in any Christian ethics or morality we might come up with. Two wrongs never make a right, and though it true that GOD can turn even evil into good, to accomplish his good purposes, it is not true that we humans can achieve good by setting out to do evil. We have already made the charge that Jews and Gentiles alike are all under sin.