To be fair, nobody remembers the US killing 20% of the Korean population. People remember what's geopolitically advantageous and reemphasized regularly. Most people don't remember all the people killed in the Murrah building. It's usefulness in the public consciousness is long gone. The MOVE bombings, the battle of Blair mountain, or Tulsa until very recently.
You don't need most people to remember you to be remembered and recognition changes over time, someone seen as villain can change in few months/years into hero. It comes down what are your goals, after all people will never agree on everything and will have always different opinions.
The most recent example is certain Eastern European politician best known for corruption, purging opposition, making headlines together with certain Russian president last year, now seen as the saint, because for some simpletons enemy of their enemy becomes their friend.
In the cases you've mentioned, people by and large don't form their opinions based on studying historical facts in an objective way but based on what they're told, almost exclusively by their government in these cases. Certain atrocities are highlighted and emphasized repeatedly, others are overlooked or hidden, but rarely because of how bad or bloody they were. World history is basically the history of war and few people consider them all to be bad.
The only school shooters I can remember the names of are the Columbine shooters and I'd say that's mostly due to the fact they were the first in the modern school shooter era (depressing reality). Perhaps that fame helped fuel other shootings, but I can't name a single other one out of the many since then.
I'm sure the fact that we're so used to acts of violence in the US on a regular basis is a contributing factor. At this point you'd have to do something pretty horrendously bad to go down in infamy. On the other hand, if it were an act (without innocent casualties) committed against say a financial, insurance institution or some other disliked entity, they could become a folk hero.
