Here we get a picture of a watchman, the man who is out on point, looking for the enemy army who would attack. If there is no early warning system, the inhabitants of the country would be quickly overwhelmed and conquered. Ezekiel is the watchman and he is simply saying, "If you don't pay attention to what I am saying, your blood is on you."
In middle America they have these things called, "Tornado Sirens". When they go off you usually have 2 - 3 minutes before a freight train comes through your living room. I remember as a kid hearing people say, "Oh, I heard the sirens, but I didn't think that the tornado was coming here." They said this in front of a pile of wood that use to be their house.
Ezekiel then goes on to say, if the watchman doesn't give the early warning signal, then the blood of everyone who is destroyed is on his hands. Chilling thought, isn't it? Ezekiel is doing what God has asked him to do. The question is are we doing what God is asking us to do? We are the watchmen! There is coming a day when life will be over and judgement will be passed on us and this earth. Did we warn anyone of impending doom of this world? Some might say, "Well that isn't a particularly positive message to uplift people with." When I heard a tornado siren, it wasn't uplifting either. But I had enough sense to get out of the storm and find shelter.