But if someone were to open up a way whereby a positive afterlife outcome was assured simply by carrying out an act such as you describe - then I could very well imagine myself leaping at the chance. What's a few score years here when certain bliss can be had by 'sacrificing' it? What use a few score years when my sin could well lead to me blowing it and spending some or other time in purifying or permanent Hell?
So in summary - Islamic terrorists are wrong and you are right because you believe it to be so. They also believe they are right but they are simply wrong about this. Because you say so.
Additionaly you would do exacly as they are doing if it would lead to a 'positive' outcome. Again Islamic extremist do beleieve they are justified by their belief and that the outcome is positive but they are simply wrong. Because you say so.
This world is a dangerous place because of people like you.
"God" help us all.