Yes, I think the world should be alarmed. The fact that we know little of North Korea, it refuses to adheres to global sanctions and continues to be a threat to South and to the surrounding nations should be a cause for concern. How blatant it shows its military might and how blatant it shows that it can do as it pleases as the United Nations all fail to think of a way to placate Kim Jong - Un. He's easily provoked and to fear of war, the United Nations acquiesce to its actions. That is what worries me the most.
It depends on how you see them and their attacks. Many countries see them as empty threats, and although they always attack threaten neighboring countries, there has never been real ones, which I am hopeful would not happen because it can spark a war. However, I am concerned with the fact that North Korea is I think may be secretly backed by either Russia or China, both superpowers than can go toe to toe with the United States. I actually don't know what they are trying to prove or what their goal is, especially considering that their nation is not the richest.