Before continuing, it should be noted that this topic is discussed among theologians, but there is no official teaching which all Catholics are bound to. That's true for most questions which begin with "Why doesn't God just?" This is especially true in a specific case.
Your question is one of theodicy. It can be made briefer with "Why do good things happen to bad people?" And you would not be unique in noticing that. Christ says that rain falls on the just and the unjust (Matt. 5), and a sizable chunk of Ecclesiastes is dedicated to that idea. For that matter, much of Jeremiah and Isaiah point to the fact that the sinners go free while the just are condemned, and the book of Wisdom has an entire chapter (16?) on it too. Then there are the Psalms, which include such lines as "My God, my God, why have you forsaken me? (Psalm 22)."
From a biblical perspective, the answer is easy:
When he opened the fifth seal, I saw under the altar the souls of those who had been slain for the word of God and for the witness they had borne; they cried out with a loud voice, "O Sovereign Lord, holy and true, how long before thou wilt judge and avenge our blood on those who dwell upon the earth?" Then they were each given a white robe and told to rest a little longer, until the number of their fellow servants and their brethren should be complete, who were to be killed as they themselves had been. (Rev. 6:9-11)
It seems difficult to provide direct citations of theologians which treat on "Why are there sinners?" while excluding "why is there sin?" From this Wiki entry one can gain two rather important points (using Wikipedia because it summarizes and I'm having difficulty finding direct quotes).
- Evil is allowed as a product of free will (and that includes evil to others). This is the natural bi-product of the Augustinian position.
- Evil is allowed because of a greater good (a Thomistic approach). This would imply that ISIS is there because it will lead to a greater world that had ISIS not been around at all.