Why did North Korea become poorer and apparently more brutal than other socialist states?

First, define your terms. Is North Korea really a "socialist" state? If you read the North Korean constitution, the world "socialist appears frequently and Article 1 says that "The Democratic People’s Republic of Korea is an independent socialist state representing the interests of all the Korean people."(bolding mine) But really, the more important word in their constitution is "juche". The preamble states, "The Democratic People’s Republic of Korea is a socialist fatherland of Juche which embodies the idea of and guidance by the great leader Comrade Kim II Sung." The juche philosophy is the bedrock principle of North Korea. The basic idea is self sufficiency. Historically, Korea had always been trapped between the more powerful Chinese and Japanese groups and, during the Joseon dynasty especially, had become the "Hermit Kingdom", attempting to avoid contact with outside groups. Southern ports still traded, but the general rule was to keep outsiders out. The Juche philosophy follows that same basic idea. The North Korean people will trade with outsiders when necessary, but they want to be as self sufficient as possible.

The other policy that is critical to understanding North Korea is the "songun" policy, or "military first." This policy came about under Kim Jong-il, placing the needs of the military above other sectors of society. Part of this is attributable to the military as KJI's source of power and part of it is attributable to a genuine (whether warranted or not) fear of US attacks on North Korea. Around the time that Kim Il-sung died was when the nuclear issue really began to heat up and there was talk in the Clinton White House of pre-emptive strikes against North Korean nuclear targets.

These two ideas, juche and songun, are critical to understanding the massive failure that is North Korea. At the end of World War II, North Korea was the more technologically advanced region. The Japanese had built massive factories, hydroelectric facilities, and other infrastructure in the North. The South was the breadbasket and besides a decent railroad system ended up getting the short end of the development stick. Today we look back in amazement at returning Koreans choosing to go to the North, but at the time, living in North Korea was, arguably, a better proposition. Of course, all of this changed with the Korean War with North Korea being bombed mercilessly by the US Air Force. After the war, both North and South Korea received monetary and material support from the USSR/China and the USA, respectively, and both countries began rebuilding.

Your question asks "why did North Korea become poorer," implying that other socialist states didn't do very well either and this is where I'll tie it together. Korea, as a whole, does not possess a lot of land OR natural resources. North Korea does have raw minerals and ores, but they are of questionable quality and they don't have them in, relatively, great supply. North Korea relied very heavily on support from the Soviet Union, especially with food aid and "friends pricing" on fuel. When the Soviet Union collapsed in 1991, the food aid disappeared as did the cheap fuel. Enter "the Arduous March," the famine in the mid 90's. Deforestation had led to poor planting conditions. Massive floods wiped out crops. This was compounded by the poor management that socialist states are known for and voila! - an entire generation of North Korea was wiped out. Killed by mudslides and flooding, starved by the famine, malnourished and deformed as a result, an entire generation has suffered the results.

South Korea, on the other hand, invested their money and support in building factories and importing raw materials. The 1965 Treaty on Basic Relations between Japan and the Republic of Korea got the Japanese to pay reparations of nearly 800 million dollars. The South Korean government invested this in building a cross-peninsula highway, the Pohang Steel Company (POSCO), a light manufacturing center in Guro, Seoul, as well as starting on building the Seoul Metro Subway system. Further, they employed Korean chaebols, notably Hyundai and Daelim, for the construction work. The end result was South Korea was able to import raw material, process it into products, and sell finished goods on the world market - the complete opposite of the Juche ideology.

This combination of poor resources, poor management, and poor anti-trading ideology coupled with a massive military budget and a famine is what has led North Korea to be such a poor place.

As far as the brutality goes, I'm hesitant to say that it is better or worse than other brutal dictatorships. We know they have camps, we know that whole families are sent there, and many of those survivors have testified before the UN commission looking into North Korea. Are they worse than Mao or Stalin? That's really a judgement call and I'm hesitant to make it for a few reasons. First, the sheer numbers of people who suffered/died under those two are quite high. Do we really want to get into a discussion on the value of human life, torture, and try to quantify it? Second, while I believe the defectors in general, some of their stories have been found to have been embellished, notably Shin Donghyuk and Yeonmi Park. Don't get me wrong - North Korea is a brutal place and some very heinous things are going on there. But, when some of the most notable and public defectors fudge the truth or out and out lie, it should cause everyone to take a moment and think about it. I highly recommend that everyone take a look at the Report of the Commission of Inquiry on Human Rights in the Democratic People's Republic of Korea and draw their own conclusions. I think this goes without saying, but it is a tough read.

Under the Loving Care of the Fatherly Leader is a great book for looking into the Kim family. It's a bit older though so there's not a lot of information on Kim Jong-il or his unfortunate, dead fat brother, but it gives a good look into Kim Il-sung and his PRESBYTERIAN UPBRINGING (GASP, a God fearing communist?!?) and how his son, Kim Jong-il came to power. It's not very different from Mao or Stalin and their cults of personality.

As for economic data, there is a bit of discussion about it in Michael Seth's History of Korea. It's a basic college level textbook on Korea with a basic discussion on how the two economies diverged.