Hello!
I am playing in Giran (classic), and currently at lv78. Started to play L2 around 2006 on private l2j servers.
The state of PK in the past was very different and there are several major factors which affects the current sitation.
The most common reasons behind pk which I experienced or commited in older L2 chronicles were:
Clan confrontations but 1 side was not willing to declare the clanwar, usually the other side was more powerful (member, gear, etc)
Clearing up farm spot, or taking over a raid/epic.
Bot killing.
Someone was bored as hell, he tried to enchant some of his gear, then go to a last PK rampage and quited the game.
It was not too hard to workdown the karma if you regulary done the sin eater quest and your PK count was below 10. Was easier to defend against PK because you had CP pots and manually controlled your character, so runaway, bsoe, pots + pvp happened frequently as an answer to a pk attempt.
Current reasons in Classic which I experienced:
Clearing up farm spots. 24/7.
Extortion. “If you pay some adena/euro I let you farm here for a week.”
Damage. The XP which is lost is no joke at higher levels, esspecially if someone do not have high % ressurection skill. You loose fix 4% at death.
We have autofarm, so people use it, and some who are more ambitious even use bots. With this autofarm some the population feels that the creater legalized bots in some degree.
It is very hard to defend yourself when you are not even at home, so pk is easier. Also there were cases when some bot owner group takeover a place for adena farm (like ruins of agony), they farm with 25-30 different parties, buff all of them regulary with scripted path following buffers and has shifts to defend their territory with the account shared PK classes.
There are no CP pots just hp pots which are not that effective to reset the intial damage. Usually people only want to start pvp when they are at full cp/hp and do not feel a disadvantage.
Elf summoners has a horse which has very-very high AOE damage, and also can use it while he is mounted. This class is about 95% of the PK chars which I see. It is seriously OP without too much gear, you just need to be somewhat skillfull with targeting and keep enemies out of range with speed. This class is far more imbalanced than any other which I ever seen while I played on some really bad-custom-setupped high rate servers too.
We have global GK, which let a player even with karma to teleport around the world which is actually works as an infinite BSOE, you just need some adena. So escape for players with karma easier than ever.
Players adjusted to the situation and setup as always, and the situation and setup in classic servers feels somewhat grim because of the reasons above.
Good luck with your work!