Was Kim il-Sung as bad as his son/grandson? He was much worse.

Well, that might not be fair. We can't really quantify how "bad" a person is, and of course an individual's scope for cruelty will vary depending on his circumstances in life. A sociopath running a major government post will necessarily have more outlets and impact than a sociopath running, say, a Sunglass Hut. The three men you've cited here led North Korea in vastly different times, so even if Kim Jong-un turns out to be the evilest thing that ever eviled, he doesn't have the same resources to which his grandfather and father had access. His tenure in office has been relatively quiet compared to theirs (... so far) in part because of this. Is he less "bad" than his progenitors for that reason alone?

I'll let the folks at /r/AskPhilosophy handle that one and restrict myself to the historical outcomes of the decisions these men made for North Korea. Due to the 20-year rule here on /r/AskHistorians, we'll have to leave Kim Jong-un alone for the most part, as he's really only been active since 2008-2009. We'll deal with Kim il-Sung (Dad) and Kim Jong-il (Son).

I'd argue that Kim il-Sung was worse than his son because North Korea's most serious problems and greatest cruelties are the direct result of decisions he made or approved. Honestly, pretty much all the stuff that makes people stare at NK in disbelief or revulsion is owed to Kim il-Sung and the political and economic systems he implemented.

These are some of the more malicious policies that Kim either created or had a hand in:

• The songbun system: This separated North Korean citizens into dozens of classes, divided still further into the "loyal," "wavering," or "disloyal" classes. It's basically a latter-day version of feudalism in which a substantial portion of your future is decided at the moment of birth based on whatever your family happened to be in 1940s-1950s Korean society. If your grandfather fought with Kim il-Sung against Japanese imperial forces in Manchuria, you belong to a privileged class. If your grandfather was a landlord or a merchant, you probably aren't feeling too kindly toward him even in 2017. (The effects of songbun have weakened as time has passed, and as formerly-despised subclasses like Japanese-Koreans or traders with access to hard currency weathered various crises better than most, but it's still a thing.)
• The development of the concentration camp system: After forcibly exiling many of the Soviet-Koreans who'd helped him to power, Kim realized it was safer to keep potential enemies or malcontents within the country itself. Exiling someone puts them beyond your control. Imprisoning them? Better. Working them until they died from exertion and lack of food? Even better! It saved on bullets and made the regime a little money off their labor on the way out. Average life expectancy for a prisoner sent to the mining detail was about three months.
• Punishing three generations of the same family: One of the things that made control of the country much easier was the exceptionally cruel practice of sending political prisoners' families to the camps alongside them. If you messed up, which could be over something as innocuous as a mild complaint about Kim or NK's government, then you, your parents, your siblings, and your children would all go to the camps. (Spouses were generally exempt under the rationale that they were not a blood relative.) Typically, the harshest punishment would be restricted to the actual offender, and your family would take its chances in more "lenient" camps where malnutrition and beatings were common even at the best of times. Family members were usually released upon the death of the original offender. From Kim's perspective, it was necessary to dig out the roots and branches of an unhealthy plant, lest it spread elsewhere in society.
• State-sanctioned kidnappings: NK has done a lot of coldly evil things over the course of its existence, but this has to be among the very worst. North Korean dissenters who had tried to escape the regime and were applying for asylum abroad were forcibly abducted and brought back to NK, never to be seen or heard from again. Hundreds of South Korean citizens and dozens of Japanese citizens disappeared between the 1950s and 1980s. NK kidnapped them to provide its intelligence service and military with people who could train operatives in South Korean/Japanese culture and dialect. Some of them were only teenagers when they were abducted, and most never saw home again.
• Happy/Satisfaction Corps: NK's government used to canvass schools and look for attractive young women to serve in the "Happy Corps" (troupes of dancers, singers, and entertainers for Kim's palaces) and "Satisfaction Corps" (effectively concubines). 15 was considered old enough to become one of Kim's bedmates. In fairness, Kim Jong-il later took advantage of both "corps" as well, but it was developed for his father's benefit.

North Korea's near-absolute control over the information given to citizens before the 1994-1998 famine continues to pay dividends for the now-dead Kim il-Sung. He is still remembered fondly by older defectors, most of whom don't have the historical background to realize that he was behind most of the policies that made North Korea a totalitarian state and were responsible for the famine as well. Kim Jong-il is not remembered as fondly and often gets the lion's share of blame for NK's decline, although the reality is that NK had been on a downward spiral for decades prior to the famine. We'll touch on Kim Jong-il's PR problem in a bit.

I'd argue that Kim Jong-il was not as bad as his father for two or three reasons: He is credited as the person who stopped (or at least lessened) the "three generations" approach to punishment in the late 1980s and early 1990s under the rationale that the state should not make unnecessary enemies. (However, there's some disagreement as to whether this change was permanent.) He liberalized both the economy and political system under the famine (or perhaps more accurately, turned a blind eye to the ways people coped with it) and didn't roll back all the changes afterwards. Additionally, he seems to have possessed at least a measure of the self-awareness that his father profoundly lacked, in part because he'd been one of the people responsible for kicking the personality cult into high gear as a means of sucking up to Dad. I mean, that's pretty damn bad in itself, but he publicly admitted at least once that he knew the peoples' adulation was fake.

But again, this is where we run into problems of how to assess how "bad" each man was. When Kim Jong-il was a younger man and North Korea better-protected by geopolitical circumstances, he was also the likely instigator behind the 1976 axe murders, political assassinations, and the kidnapping (or attempted kidnapping) of South Korean artists, musicians, and film stars. So did he moderate his approach as he grew older and wiser? Or did he just sense that he had less room to maneuver than his father did, and that the state he oversaw was more vulnerable? That's a question a lot of East Asian historians would love to be able to answer.

So. Why does Kim Jong-il get worse press?

• The physical difference: Kim Jong-il was 5'3" and wore platform shoes and a bouffant hairstyle to make himself appear taller. He had a reedy voice, avoided public speaking, and was not the most attractive person around. By contrast, his father was 5'9", chubby but carried the weight well, was a reasonably attractive man, had a big belly laugh, and was described by most listeners and observers as a magnetic personality. Frankly, his son just wasn't as physically compelling. This shouldn't matter in politics, but it does, and one of the things that has probably benefited Kim Jong-un to a degree is his physical resemblance to his grandfather.
• The famine: In a really unfortunate coincidence, Kim il-Sung died before the 1994-1998 famine really got underway and thus largely escaped the blame that should rightly have been his.
• The increasingly porous nature of North Korean society: Again, Kim il-Sung had one fantastic advantage over both his son and later grandson; NK was more effectively sealed off to the outside world when he was in power. The state had much greater control over the information that reached its people, and no bad news was ever associated with Kim il-Sung. Once the famine got underway, people started moving more freely throughout the country and into northeast China. With the growth of smuggling networks, cellphones, and the later penetration of Chinese-built radios, VCRs, and DVD players, it became impossible for the North Korean state to keep peddling the line that their country was an oasis of calm and good government in a world gone mad. However, the blame was assigned to Kim Jong-il, and not to the man who'd actually created the system that had failed so catastrophically.

So in the end, my contention is that Kim il-Sung -- and by extension, the many people who helped him build a murderous regime -- is the person who merits our greatest censure. We can damn Kim Jong-il for having sent his enemies to concentration camps, and we do so quite rightly, but he wasn't the person who built, staffed, and filled them.