Ignorance. They know nothing about it and that allows people to fill the gaps in their knowledge with whatever bias they have. It's also true that all people hear about are Islamic terrorists. Obviously the jihadists represent a very small number of Muslims.
I agree with you. People misperceive Islams to be bad peoples, because they are biased by what they see in the media, they only believe what the media says and don't listen to anyone else. I believe they need to do a thorough research before they can judge Islams for who they are.
I don't hate
Muslims. I also don't hate Islam, exactly. Not near as much as I hate Christianity. Both are just flat wrong, and I'm about to show you why.
This is actually true of ALL Theistic religions that I am aware of.
1. A priori beliefs:
There is a God
This being is Omniscient and Omnipotent
This being wants to be worshipped.
2. The problem.
If all of the above are true, how many religions could their possibly be?
Y'all know the answer, you just don't want to face it.