I think you just need to share some truths with them. Logic speaks for itself. There have been a lot of bad people who where Christian and Jewish but it doesn't make them all bad.
- Catholic Priest who molest Children
- Hitler was Christian
- One of the worst massacres in history occurred under Milosevic as captive nato workers stood by helplessly as thousands or muslim boys and men from 12 to 60 were systematically murdered by Serbian soldiers who were mainly Christians
I mean honestly this list could go on and on...
Basically all humans have the ability to do evil acts. 911 was more of one groups political amibitions than a religious act. They use religion to try to justify the actions and manipulate uneducated pawns into fulfilling their plans. Islam forbids the murdering of innocent people especially women and children. I believe the ones who comitted 911 would have been considered murders under Islamic law because their intent was to kill innocent people. There were Muslims working alongsides others in those buildings also who died innocently.
I am not a scholar by any means but I know from what I have read about the Prophet Mohammed (PBUH) he never would have participated in killing innocent people. He went into battle with armies against enemy armies. He was even merciful to captives. He also made it clear God forbids suicide. To be a Muslim we must follow the Prophet Mohammed's (PBUH) example of how he lived... the Sunnah.