Player killers / random killers, typically found in many MMORPG games on ROBLOX, are players that kill others (mainly) for fun or for no real reason other then to ruin others' time. Other player killers do it for a reason (which you may find in the link at the bottom of the article). They are commonly seen in any server and mainly hunt after noobs or lower-leveled players.
They are generally at Floor 2 or Floor 3 due to the high amount of low leveled players (players need to be level 5 or higher to legitimately be pk'ed). If not going after noobs, they may be after other players closer to their level just to pick fights.
As many know, pk'ers / rk'ers use highly powerful gear to power through enemies and players. For example, some use 'Breeze,' 'Divine Inferno,' etc. If you are going to go against them to defend yourself or chase after one, try to focus on both attack AND defense.
What to Do / Tips:
A lot of players die to them, but if you be careful, especially at a mid-level, you can defeat a pk'er. Beware other helping pk'ers, and try to get friends to help you (or high levels). Make sure you keep a standard amount of health. Can't fight them? Use 'speed' to your advantage, if you have items like daggers. Lastly, don't die over and over.
"In the Anime?" (Referring to Sword Art Online):
(NOTE: Swordburst Online is not fully referring to SAO, but some assets are references to the anime overall such as the Holy Excailbur, Dark Repulsor, etc. Info from the anime may not be the same in SAOB.)
In Sword Art Online, player killers are very hated, and they are seen throughout the episodes. They are commonly disliked amongst players such as Beta testers and (sometimes noted) level hiders. There are also players of SAO that have been in 'the Laughing Coffin,' a murder guild. If you were die by one of these or any other enemies, you would die in real life. (Of course, this does not apply in SAOB.)
For even more information on the topic, review the main post about Player Killing: http://saob.wikia.com/wiki/Player_Killing