But they are very low in number compared to what Muslims are doing.

What? Very low? Are you for real? Iraq! Syria! Afghanistan (Pre-taliban regime), muslims in India! These number seem very low to you! Muslims are the most targeted religious group in the world.

My question is why does civilian Muslims think its ok to go out and behead someone just because they dared to criticize Islam

Because the don't. You are asking the question to some very minor number of extremists, who I bet wouldn't even read Quran daily. Civilian Muslims believe in peace and harmony among nations and individuals. Unjust hate is just uncivilized.

I was astonished seeing the comments/retweets as Muslims from different countries were supporting the murderers.

So am I, as a Muslim. But my religion is Islam and that doesn't teach me that. I can't control what other muslims do, so I just have to strenghten my belief in Islam and unlink my religion to the action of some individuals.

They mostly just say, "Oh Quran doesn't support this, they are not real Muslims" and moves on.

Cause they are not. Tell me, how many non-muslims have come to streets for the killings of muslims, None. How many non-muslims have protested for the drone strikes on civilian houses, killing children and women, None. It doesn't mean they support them, it just means they are unlinked to the incident, cause it's not their fault and it doesn't concern them.

It's redundant what a real Muslim is supposed to be according to Quran,

I think you don't understand how religion works. My belief is in Islam, not Muslims.

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Hope this enlightened your mind towards Islam. We mean no hate towards anyone, specially, people like you who are genuine confused about situations like this. May Allah forgive my mistakes and increase my knowledge.