It's too difficult to say "player killing is wrong except in these cases". You either have to say it's wrong in every case - and ideally follow-up by also making it near-impossible to do anyway - or you have to accept that because there's too much context to fit in without having human judges and incident investigators running the justice system, it can't be treated any more severely than you'd want to treat the least severe case.
Three players are in a system - Terry the trader, Bob the bounty hunter, and Paula the pirate. To simplify, let's say that they all know each other by reputation, but none of them have any relevant bounties at the start of the day. The order of ship power goes Bob > Paula > Terry but all three are armed and capable of fighting, so it's not guaranteed that a combat win goes that way every time. In this context there are cases where a simple "if the attack is in-game illegal it should result in severe in-game punishment" approach is going to cause problems.
Case 1:
- Paula interdicts Terry, and demands some cargo. Terry, having seen Bob hanging around in supercruise too, reckons that help is on the way and refuses.
- Paula deploys hardpoints, Terry does likewise, and both open fire.
- As it happens, with both dodging, one of Terry's shots lands first
- Paula is still clean at this point, so Terry is going to get landed with the PK punishment (and Bob, despite knowing Terry is the "lawful" pilot, has much the same choice on their arrival - opening fire on Terry is fine; opening fire on the still-clean Paula will get him into trouble too)
Case 2:
- having successfully intimidated Terry at gunpoint into giving up some cargo, Paula is heading to a black market to cash out
- Bob sees Paula is wanted for assault and interdicts, opening fire immediately
- if Paula returns fire (an entirely reasonable thing to do; even NPC pirates will in similar situations!) she's going to face a PK punishment if Bob doesn't know when to quit
Case 3:
- after their last encounter, the next time Paula interdicts Terry he just dumps ten tonnes of cargo and runs
- Paula shrugs and starts picking it up
- Paula now has a fine for the interdiction, but no bounty (and even if she's clumsy and gets scanned coming into the station to sell up, it'll still be a fine)
- Bob arrives on the scene in time to see Paula pick up the last of the stolen cargo
- Paula grins and calmly wanders off to find another trader, knowing that Bob can't legally open fire
Case 4:
- the Thargoids have invaded the system, and Bob, Terry and Paula temporarily put aside their differences to grab an AX combat ship each and defend the station
- in the confusion of battle, some loose shots inevitably hit the others
- when Terry gets the wrong end of a Medusa's full firepower 10 seconds later, Bob ends up with a PK bounty.
Case 5:
- Terry is concentrating too much on avoiding Paula's interdiction attempt, and messes up his glide approach to a planetary port, coming in about 40km short
- Paula follows him down and opens fire, trying to grab some cargo before the system authorities show up
- With Terry not dodging as much as usual between the gravity and the need to get in a straight line quickly, a full round of fire from Paula hits the aft of his ship...
- ...and the shields go, shortly followed by the drives.
- Terry reaches for the reboot/repair switch, but it's too late
- technically it was the ground which killed him, but Paula still gets the blame.
The game is not sophisticated enough to implement rules of engagement to the degree needed to distinguish between "good" PvP and "bad" PvP or correctly assign responsibility for kills - and while alternative systems might adjust exactly which cases give intuitive results, they're always going to have odd cases - some of which, like the infamous Sidewinder with 1% hull, might be convenient enough to be weaponised. Even the fairly token punishments for player hostilities at the moment are sufficient to make you very unpopular if you leave crimes on in an AXCZ, for example.