Well, let me tell you that I really don't have anything against islam per se. It's just that I'm concerned how easily it can be interpreted in destructive ways.
Most religions can easily be interpreted in bad ways, especially if you remove context, which a lot of radicals tend to do. Heck, look at the Crusades. Anyone who's read the New Testament would understand just how absurd the Crusades were: they were basically wholesale slaughter and every other manner of evil targeted at Muslims in the name of Christianity and reclaiming the holy land. Does anyone really think Jesus would have endorsed such a thing?
I'm in a bit of a hurry here, so hopefully this makes sense.
Ideally, everyone would base their ethics on systems which can be rationally constructed. Now, we haven't found a perfect, rational basis for morality yet, and we may never find such a thing. People even disagree strongly on the different systems we've come up with. But these systems we've produced are, for the most part, clear, easy to follow, and easy to understand if presented correctly. And they tend to provide the right answer for deciding how to act in nearly every case (most of the problems come from thought problems, extreme cases which almost never happen in reality). Moreover, the discussion is something that anyone can contribute to, given sufficient interest and expertise. The philosophy of ethics is a huge field with many, many people continually working toward a better, more useful answer to the question of "what is good?"
But this isn't an ideal world. Religion's a real thing that matters to a lot of people. If it's a force of good in their lives, I see no reason to upset that balance. When it's used as an excuse to do evil, though, that's another story. I think our best option there is to point out how their behaviors conflict with what their religious texts say. If that doesn't work, then appeal to the common basis for ethical understanding that nearly all of us have: most of us understand at least some subset of human behavior to be wrong, and these are things that can cause some clear, measurable harms that we can point out. Normally, I wouldn't suggest appealing to someone's emotions, but sometimes a rational argument isn't enough. Pointing out how much pain their actions can cause might be enough to move some of these people away from the path they've decided on.