Griefing has been around forever, not just in online games, but
in real life as well. It's just that in online games, you're less
accountable and punishable by law (or by gangster friends of the
victim looking to beat you up), so you grief others more.
In Ultima Online, the 1st MMORPG (released 10 years ago, and
still running strong today on both official EA servers as well as
non-official private run servers), there have been griefers
since 10 years ago. Griefers and PKs are two groups that
overlap/intersect, but are neither mutually inclusive or mutually
exclusive. One can be the former but not necessarily the latter,
and vice-versa. But griefers are universally hated more than
PKs.
Griefing in real life also exists, for sure, in every strata of
society. From a power abusing boss bullying a subordinate he
doesn't like, to underworld gangsterism/mafia/yakuza, to a family
member deliberately making life grevious/miserable for another
(usually re-acting out issues of an inter-karmic prison that
cuts across lifetimes, unwittingly and unconsciously), to political
powers griefing political victims, etc.
Online, in games, on forums, griefing is altogether more common
because it is easily done, and most pple who don't dare to do it in
rl, but have the griefing urge within them, will use the
opportunities online to do so. Games present even more
opportunities than forums, because in games, you can kill each
other, and do stuff (within the game mechanics or rules) that can
make life miserable (or grievious) for other players, without much
penalty, if any at all.