Nobody's perfect. Let him who is without sin cast the first stone (John 8:7, paraphrased). Why do you look at the speck of sawdust in your brother's eye and pay no attention to the plank in your own eye? (Matthew 7:3, NIV)
These sentiments seem to indicate that we shouldn't criticize others because we are imperfect too and we have committed sins in our lives so how can we justify being critical of others. After all, if we haven't got our own affairs in order, what makes it okay to tell someone else they've got a problem.
But that doesn't really work, does it? When you have a disagreement with someone or you see a particular behaviour in a loved one that irks you, it's important to have a conversation with them and let them know how you feel. If their behaviour is affecting you in an emotional way or there is a more concrete manifestation in your life of their problem, then you have a right to be heard and even to help that person to see the error of their ways. Right?
That's what an intervention is, after all, an opportunity to explore the shortcomings of a loved one because it is now affecting you in a very real and negative manner. Generally, you've recognized that you can't take any more, or that the person is genuinely going to harm themselves so you want to accomplish two things. You want to help the other person and you want to remove the stressor in your own life.
A conversation or intervention doesn't always go smoothly. Let's face it, you are hi-lighting a conflict that the other person may not even realize exists.
It makes sense that when you want to "talk" to a loved one or friend about something that's been bothering you, they might become defensive and immediately point out a flaw in your own character. They are really just pointing out the obvious, that you are not perfect either, that you shouldn't stone them since you are a sinner too, and that there's really no reason to take exception to their issue when you have issues of your own that you haven't dealt with.
Perhaps it's the root of all conflict. You are pointing out something you perceive is wrong with me so I'm going stop you, even if I have to take you to task on everything you represent. Hey, I might even kill you because then this conflict will stop altogether. Wait, there are more of you who feel this way so I need to either admit I might be wrong or kill more of you. I will never admit I'm wrong so, killing it is.
Cognitive dissonance at its best (worst?). Then again, nobody's perfect.