Let's say that someone shows you some art they have made, and asks you to comment on it.

If it is good, and you tell them it isn't good, you have insulted them, because you are wrong. If it is bad, and you tell them it isn't good (tactfully) you haven't insulted them, even if they are offended, because you are justifiably telling the truth. If it is bad, and you tell them it is good (not just politely), you have insulted them even though they are happy, because you have been dishonest and flattered them or treated them like a child.