I understand perhaps how they justify killing those who don’t believe in their religion, but how is blowing up people of their own religion justified?
Like what would bin Laden say if he were asked why his organization kills muslims as well as "infidels"?
Osama Bin CrapBumb and other so-called Islamists have responded to such questions by claiming that the Muslims they do kill are no better than the ‘infidels’ to use their word since they work with them to fight against their terrorism. They justify killing their own brethren by proclaiming them ‘traitors’ and ‘false Muslims’ basically.
Do you know how Bin CrapPoo justified killing Muslims in the 9/11 attacks (Muslims were murdered there too you know)? He said that it was ok because they had ‘voted’ for President Bush and were therefore also responsible for the crimes Bush committed in Iraq and Afghanistan 
What twisted logic aye?
I don’t call those people my ‘brothers in Islam’.
Lastly, blowing anything up would be forbidden according to Islam as it is a form of indiscriminate killing which Muslims are not allowed to take part in. Heck, Muslim armies during Muhammad’s (pbuh) time weren’t even allowed to hurt trees and animals, let alone kill innocent bystanders!
Muhammad (pbuh) would have never permitted gay suicide bombing if it had been around in his time. But Islamists don’t care about Muhammad’s (pbuh) life and character. In their mind, they are right and everyone else is wrong.