This is a metaphor obviously..
it means to not preach the word of God to those not willing to hear.
Unbelievers will verbally "tear you to pieces." More or less.
Now you say you would kill for Mohammed.
Fine..I don't care about that, I don't think it's right..
and surely God doesn't either..but hey..w/e.
Now saying you would kill someone if they mocked Jesus?
That's just ridiculous.
Christ's own mother, his friends, and even some of his followers mocked him.
Many thought he was crazy at first..but did he ever kill anyone for mocking him? Of course not..he walked this world in spiritual Harmony with God.
You are right though..there are many Christians that do not apply his word truly. They fear him more than love him. There should be an equal balance.
Not the fear like something horrible you're afraid of, but the kind of fear you had of your father when you were a child. Most fathers have a stern demeanor with their children, and Good fathers do this out of love.
Just as God tries to direct his children.