Maybe they can stone them to death, or even behead them and tape the beheading to cast fear in the hearts of terrorists. They could cut the ears and tongues off women who are believed to be involved with terrorists. Then, they can hack to death the children of the terrorists to teach those extremists a lesson.
You want moderates to behave like extremists in order for you to believe that they're not terrorists.
That's just dumb.
They need to behave in a civilized manner and bring them to justice, executing them as proscribed by secular law.
When your government won't deal with monsters, vigilantism IS the civilized approach.