The Bare Bones Message
The message we need to get across is simple, but it's still too much for many people to emotionally cope with. It ruins the life they once had. It shatters their innocence forever. Because of this, they will immediately try to find a way to dismiss this simple message. The first and easiest thing to say is, "Other religions have violence and intolerance in their texts." If you have no answer to this, they will be relieved. They can safely put the issue back out of their minds. "Everything is okay," they'll think, "Islam is just like other religions." There are other versions of this same objection. For example, Christianity is just as bad, all religions are evil, there are extremists in every religion, etc. (We have answers to these objections here.)
However, if you answer the objection successfully, the next thing people will usually come up with is, "Most Muslims don't follow the texts." That's somewhat true, and it's a good thing! But it doesn't change the fact that the texts themselves are dangerous to non-Muslims. It doesn't make the problem go away. Other versions of this are: Only extremists or fundamentalists take the texts literally, not all Muslims are terrorists, I know a Muslim and he's really nice, etc.
Just like getting any horrible news, people will try to deny it. That is the first line of emotional defense. They'll try to find a way to escape from the terrible news and its implications. But if you can prevent someone from escaping the hard fact, you will have done all you need to do. Reality will teach them the rest.
There is a lot we can say about Islam. But it's good to be clear about the most important fact to get across, and to stay focused on it: What is written in Islamic texts is dangerous to non-Muslims. Get this to sink in.