I'm just gonna LIE down...

Well, I believe that the reason why Jonas finds the instruction about lying so disturbing is because he has not done it before. Jonas and everyone else in the community, are too used to obeying the rules, following them to the letter. Of course, there are little white lies, like the underaged children learning how to ride a bicycle before their Ceremony of Nine. But, overall, nothing majorly serious.

In Jonas's daily life, from the moment he wakes up till the time he retires to bed, he is constantly reminded that he needs to speak the truth at all times. This has been an overriding principle ever since he was born and Jonas believes that lying goes against the very institution of living in such a structured community. It is not surprising that Jonas's reaction at being permitted to lie was one of being incredulous because the analogy would be for someone in the modern world to be given the license to kill. Deep in his heart, Jonas hopes that it would never have to come to this.

But as the story progresses, he finds that lying has become a necessary evil. He needed to maintain a calm exterior for his parents' sake while his heart and mind underwent tremendous changes. He reasons that the most convenient way to resolve his inner conflicts is to by way of lying which was difficult initially, but became almost natural later in the book.

"He killed it! My father killed it! Jonas said to himself, stunned at what he was realizing." (page 150 ) Ultimately, he rationalizes that adults around him lie to maintain society's order and that he being the receiver, has every right to become the greatest lier of them all.