Is lying/not telling the truth really a bad thing?

Yes it is. No matter how benign your reason may be, lying is a bad thing. Sometimes people claim that lies are sometimes required to sustain friendly relationships. Complete honesty may sometimes harm a relationship. However, the truth is, if another person cannot forgive your mistake, he/she might not be worth to remain as your friend. Sincere relations between friends have to be of candid honesty. It might be that your friend could have taken the truth, and yet, he cannot accept the fact that you lie to him. Attempting to accomodate the feeling of the other can sometimes backfire. I had a failure experience that testifies to this extremely well. Once, I had a sort of gathering with my friends. Yet, my best friend scheduled to leave Taiwan at that period of time. Thus, I decide to not tell my best friend about it, afraid that he would think I planned to isolate him from the start. I felt that since he could not come anyway, there is no point in inviting him or telling him. However, somehow, he found out. He was not happy that I did not tell him. He know he could not show up, but he cannot accept that fact that I decide not to tell him about it and pretend nothing happen. That is when I realize, there is no such thing as good lies. Good relationship should only rely on pure honesty.