Why did Samir want to blow up the people who were marching for his cause

Nobody said terrorists were brain surgeons.

He was insane.

Elise Kraft / Sharon Bridger said it was his cause, but I don't think he gave a *beep*

It wasn't his cause. His cause was obtaining the release of his religious leader who was captured at the start of the film.

I think to show his perceived folly of their ways - that no amount of talk and dialogue could "fix" centuries of controversy - and it would make the biggest impact - their goal - the terrorist goal is to inflict the most damage with the least cost outlay.

because all muslims are evil

I think because it would have drawn a lot of attention and he thought it would turn all of the marchers into martyrs...