Persistent Aggro
As a player killer, even if you are not part of a clan, you are not absolutely safe from players 5 or more levels above you.
Certain PK-related actions you may take can give them persistent aggression, or paggro as it is usually known. This means that for the rest of the boot, they can attack you just like any player within their usual level range.
Please note that it does -not- mean that they will automatically be attacking you, and in fact they may not realise that the paggro has been granted at all.