Unless you haven't watched they news lately, North Korea is stocking up on weaponry, and trying to make South Korea and the US afraid of them. Their leader, Kim Jong-Un is a young leader with nuclear ambition, and he has no idea what he is doing! Based on what they are doing, you can tell that they are obviously preparing for war against either the US or South Korea.
North Korea is ruled by a dictator with an iron fist who thinks he has the power to do whatever he wants. If they are successful in building a nuclear arsenal, they will likely use it to threaten other countries with demands and try to take over neighboring areas. As long as they have weapons and a power-hungry ruler they are a threat.
It is difficult to decide whether or not an attack will occur. Although North Korea has been practicing with the weapons that they have in their possession, this does not necessarily mean that they will ever attack. I feel that they are trying to prepare themselves in case of an emergency. Any other country could, potentially, do this as well.
Being that South Korea and the United States are allies, it would be assumed that attacking South Korea would bring the United States into war with North Korea. However, being that China would be brought into this, it's unlikely that it will happen. We are heavily involved with China, and North Korea is on somewhat peaceful terms with them. Unless North Korea made an enemy of China, it's unlikely that South Korea and the United States will be attacked.
No, North Korea will not attack South Korea or the United States in the future because North Korea knows that attack will spell doom for the North Korea dynasty. I believe that North Korea launches nuclear tests not only to show the world their military prowess but also to show their citizens that they are strong nation.