Right, so.
Nabu was killed because killing him achieved a good (freed Giovanni Zatara) and because Nabu was continuously committing evil (enslaving Giovanni Zatara.)
Paul's personal attachment - and the fact that Nabu expected to be treated as a Justice League hero - is what convinced Paul that the good Nabu did was not worth the moral compromise to the whole League.
Let's talk about that 'moral compromise' for a bit, before people shout about Adom.
Adom does not claim not to kill; he believes and wishes to be considered a net good to the world, but he is willing to re-examine himself to ensure that is true, most especially because he is willing to go against the normal code of conduct for heroes. That is to say, he is willing to hold political power, and he is willing to kill the outrageously guilty in combat.
The Justice League, meanwhile, wants to go beyond that and be essentially incorruptible - they want to go so far as to not hold any political power themselves, and to not even kill the guilty if they can avoid it. This means that accepting Nabu and, worse, concealing his crimes is a betrayal. I have heard elsewhere and support myself that hypocrisy is one of the clearest sins even when it is not the greatest, because even the hypocrite agrees that they are in the wrong.
But more importantly than that, the Justice League does not engage in self-examination very much, if at all. It took prompting from Paul before they brought Paul in to examine them - without him, they would not have done so. Any compromise the League makes is magnified because they lack the self-awareness to correct for it.
Okay, now that gives us a bit of context about Nabu, but what about the Gordanians?
Well, the number one thing here is pretty simple: Gordanians are not born evil. As far as we know, no species is. One could argue that demons are... but that requires a rather tortuous definition of 'born', since they're drawn from evil souls and then further aren't considered demons until they're sufficiently evil, and Hell engages measures to ensure that remains so.
Going back to Gordanians then: The ships Paul is on have Gordanian children, so at the very least he's not interested in killing all of them, and brute destruction of the ships will kill a lot of innocents as well. If he were going to tolerate so much collateral death to innocents then it's more tactically sound to just kill the Tamaraneans too, since trying to evacuate them gives the Gordanians warning they could potentially use to fire weapons at Paul or the planet.
Additionally, Paul has now confirmed that there are other ships, and these other ships are dangers to Tamaran, and so he has more investigation to do before he can keep Tamaran safe from this clan's military. This means that tactically speaking, killing the Gordanians here immediately is an even worse idea until he can get that information, or at least buy enough time-before-the-Gordanians-react to put up more thorough defenses.
In this situation, I think those concerns make it pretty clear that 'just killing the Gordanians', here, now, will neither protect Tamaran as a whole nor protect the innocents (Gordanian and Tamaranean) who would be caught in the figurative blast radius (probably dead to vaccuum exposure, really.)
In general, the question is a little bit harder in some ways, a little bit easier in others. If the Gordanians can be cleanly sorted out - both from their own innocents, based on degrees of participation and guilt, and from the Tamaranean slaves-slash-hostages - then the question more or less falls to "Should the guilty be punished with death, or is there something more appropriate to be done." If Paul has that kind of control over the situation, then they can be handed over to Tamaran to be executed with the understanding that this is war not murder (not a big change, but slightly better) or more likely, sent into exile somewhere they can't cause trouble. There is also the option to have them ransomed or sold to other Gordanians or another faction in exchange for resources which can be used to aid Tamaran instead of used for whatever that faction would have done with it, but Paul would probably prefer not to do that - even if it makes utilitarian sense in a vaccuum it would encourage slavery in Vega which is going to be difficult enough to halt as it is, creating unpleasant negative external-factor utility.
In-between, with neither no-sorting nor perfect-sorting, there may be opportunities where other answers make sense - killing a large number of Gordanians, or even an entire ship innocents (Gordanian and Tamaranean hostages included) in order to prevent a kinetic drop on a city, for example. But in general, killing them just isn't that helpful; cases like the Joker and Captain Nazi are severe outliers and cases like the battle in Shiruta have significant additional restrictions (time, in that case, since the soldiers were at that very moment killing innocents as fast as they could.)
There are times when killing is appropriate, but the Orange Rings are so capable of focused, flexible power that they are very rare for Paul.
If he had a couple of guns instead of the rings, things would be different. But he's got TWO POWER RINGS, so really killing people is generally just lazy, not prudent, moral, or ethical.
