It's us, Muslims.
One day somebody met Buddha and asked- 'who can destroy your teachings?'
Buddha answered: 'nobody can do that except of my own followers'.
And this is exactly what Muslims do to Islam.
WE destroy Islam from within.
First of all, we destroy Islam in out hearts.
Every time we feel superior- we destroy Islam. I think every time we feed our egos -nafs- we kill Islam in our hearts.
We destroy Islam by forming different groups that consequently fight with each other, claiming their truth is more 'truthful'. Well maybe discussing differences is not so bad after all, we are all given brains and free will.
But nothing justifies killing people who do not agree with our 'truth' no matter if they are Muslims or not.
Nobody can destroy Islam, unless Muslims give a helping hand.