From my experience with children, especially young children, the simpler is better. If I was in your situation, I would explain to the children that some people make bad decisions. I would explain that some bad decisions are worse than others (like throwing a toy in the house is “bad decision”, but choosing to hurt somebody else is a “very bad decision”). Then I would explain that the terrorists make bad decisions to hurt other people because they don’t know how to use their words to get what they want. They believe they have to hurt other people to get what they want, which is not true, I would tell them. You could even say that they never learned how to talk about their problems (or their parents never taught them to communicate their needs). If they ask what they want, you could say something along the lines that they want us to live like them, but we don’t want to. You could go into as much detail as you deem appropriate for their development if they ask how they want us to live like them. If they keep asking why they make bad decisions, and they can’t understand it is because they don’t know any other way to get what they want, you could say that sometimes we don’t know why people make bad decisions, they just do. This is true, sometimes we don’t understand why people make the choices they do.
Explaining it in this way will help give children a sense of control over their lives (if they make good decisions, then good things will happen) to help prevent them from getting scared. This is important especially for young children who find it hard to understand complicated topics, which in turn makes things scarier.
Also, you may find that changing your tone of voice may help your children find peace with the situation. If you aren’t confident in what you are saying or your tone of voice has a lot of inflection, then that gives the children a sense that there is more that they don’t understand. If your tone has confidence it may help give them some finality. Just be careful that you aren’t using a disciplining tone, because that would be counterproductive and discourage your children from communicating and asking questions from you. Just confident like how you would speak if you were trying to convince an audience something.