1. Because 95% of all terrorist attacks around the world are committed by Muslims, whether against the West or other Muslims. Sunni and Shia blowing up car bombs in front of each others' mosques for example, prove that even in all Muslim societies, Islam is violent. And both Sunni and Shia kill more moderate strains of Islam like Almadiya and Sufi.
2. 90% of all honor killings are done by Muslims, despite being less than a third of the world population. When women are routinely murdered by relatives for not conforming to Islam's oppressive religious mandates for women, like trying to choose their own husband or not wanting to veil, we shouldn't be surprised when the same religious people kill non-believers.
3. When Muslims commit terrorism, the majority response of Muslims is "they aren't really Muslim" even as the terrorists praise Allah and quote the Koran. Or they say "don't look at me with fear, you evil bigot". But they are NOT turning in the terrorists plotting, with a quarter of Muslims in the West admitting they wouldn't turn in terrorists. More than half say that Shariah law should be imposed in the West, regardless of the traditions or will of the local people.
This means a majority of Muslims either actively support the terrorism or are too afraid of their neighbors to act. When your neighbors either actively support through fundraising and endorsement or tacitly support it through silence, yes, the entire group can be seen as culpable.