The issue is not us can claim to be representatives of Islam. If you show goodness, non-Muslims will assume you might be cherry picking for all you know while ISIS is interpreting Islam more literally and sincere to the texts. There is no way to prove it by simply claiming Islam actually teaches this and not their way.
The true way of submitting to God, every religion has their own followers with various views on that.
Islam is a mess as a result but the true religion remains that of Islam still.
People will never see your good as a result of Islamic teachings, so to follow Islam a a result. They will see it these days, that in spite of Islamic teachings, you are being more human towards other due to humanistic values and nothing to do with Islam.
And more often then not, they are right because those characteristics are universal practically in all societies among many good people.
Does Islam really condemn terrorism? I believe so. But of course I can't tell a non-Muslim that this the only right way to interpret the religion, because to them, the issue of right or wrong way is irrelevant because Islam to them has not been proven to be true. They maybe believe you ought to interpret that way if it was true, but how can those interpreting it the other way be said not to be part of the religious teachings.
That said, your post is an important perspective for non-Muslims to consider. Had people actually followed the family of Mohammad....would this mess of ISIS and what not, exist today?