It's more like when it comes to BGS manipulation, it's ultimately a game of filling or draining the PvE buckets you have for each faction in a faction: influence, security, and economic. Influence decides things like wars and asset control. Security and wealth decides what types of missions are available on the mission boards. Some actions fill buckets, others drain them. If you commit murder, whether it's done against NPCs or players, it drains the infuence and security buckets of whichever faction is in control.
At the end of each BGS tick, the buckets of every player who performed
any action in the system, attacker, defender, or neutral, are weighed, some math is done on them, and the results applied.
If you're attacking a faction, and have
no interest in promoting other factions to replace them, the
easiest way to do this is to go on a killing spree. NPCs or Player, it doesn't matter, just kill everything that moves, and watch that influence tank.
However, if you want to
promote another faction in its place, that's not the best strategy. Being wanted, and especially having high notoriety, makes your agenda much more difficult to advance. You want to be able to operate legally in that system, so it's in your best interest not to go on the murder-hobo route. A few extra-judicial killings when there's a mission for it, sure. But murder sprees, no.
The idea that PvP murder can be used to
defend a faction stems from the idea that what the "defender" loses in influence will be offset by what the "attacker" potentially loses by being unable to complete the missions (or other actions) they're running.
That's only true if the "attacker" isn't expecting opposition. If the "attacker"
is expecting opposition, they can switch gears and accept incoming missions for the faction they're
attacking. Being able build reputation with the controlling faction provides an "attacker" more opportunities to mess with that faction in the long term, even if it comes at the expense of slower influence gain. If you get murdered via PvP instead? Even better, because that murder comes out of the "defender's" bucket, rather than your own. It's a "heads I win, tails you lose worse" scenario.
This scenario was even worse in BGS 1.0, because an "attacker" used to be able to negatively affect
any faction, even the one they're working against, by choosing the right missions. It's a pity that Frontier removed that level of nuance when they fixed how faction states worked.