So, if I'm reading this right, the Hijvin were becoming Slaanesh and it took one of the oldest Uninvolved going full Khorn to stop them.
This misses quite a bit in translation, not least that the Uninvolved whose perspective you saw wasn't acting out of rage. They were acting out of what was, to them, dreadful necessity. They were still very much in control of themselves - had they not been, they never would have restrained the attack form they used to annihilate the Hijvin. And an Uninvolved going 'full Khorne' as you put it would never have offered an apology.
This one did.
No, just that the reaction would be significantly less potent than if the action had been done by/to the Shiplords.
This misses some things in how emotional response is observed to work in humans - after all, if not being a party an act is done to/by would reduce emotional impact, then the Holocaust never would have been seen as such a horrific in the first place, let alone continued to be seen as such.
Humans are capable of exceptional levels of emotional response purely from observation of a situation, this is why charitable groups often highlight the horrors that large portions of our species go through every day in their attempts to get the money to stop it.
From that perspective alone, expecting a lower grade response due to being on the relative sidelines (the massive war that wiped out entire star systems notwithstanding) would seem flawed.
Consider how the Shiplords referred to the Sphere, they called them "our last true Enemy". That means something.
Here is a fun question: Does that count as masturbation or sex?
Sex. The description of the film from Wiki makes that connection easy to draw. The chip became sentient, therefore it's another life, therefore it's sex with another being - the specifics are kinda irrelevant beyond that.
Now I'm going to take a moment to talk about timescales. Humanity has seen a general measure of galactic time measurement that is referred to as 'stellar cycles', with a length roughly two thirds again that of human years. All races among the Group of Six as well as the Shiplords themselves appear to use this standard measure.
Humanity continues to utilise the Earth year as their primary form of long-term measurement, but that's likely to change - if for no other reason than simplifying things in translations. Syncing up logistical networks and the like works much better when everyone's time-units are the same.
But these are units created by species that started out with set lifespans. That approached life from the perspective of "how long do I get" in their cultural beginnings. It makes sense, of course, to measure your life compared to the movements of planets around their primaries. Where exactly the original Stellar Cycle came from is something of a mystery. It's suspected that it's of Shiplord origin, but no one outside of them appears to know for sure.
The Uninvolved, however, are beings that exist across a truly monumental scale. Exactly what they do with all their time hasn't been explained, though humanity now knows that there are significant groups within them that support and oppose the current state of galactic affairs. That's not important here, though. Uninvolved come into existence by essentially accepting death and by that acceptance transcending it - the matter of their slow fading as they reach truly ancient years is a cause of much curiosity for the younger Uninvolved, but no fear. Maybe it's death, maybe it's something else, but none of the elder Uninvolved have ever run from it as far as Tahkel could tell you.
Of highest import here, however, is the scale across which such beings live. Most races endure under Shiplord hegemony for six to ten thousand years under the current system - the Sarthee are somewhere between those figures at the moment. An Uninvolved can expect to persist for hundreds of thousands of years. This has a predictable knock-on effect on how they interact with time.
There are two polities in all the galaxy that may contain examples of sentient life older than an Uninvolved. One of these is the Shiplords. The other are the Neras.
To reduce this: consider the Asari in Mass Effect, with their ability to plan across centuries compared to decades. Does time truly have the same affect on them? I doubt it. Now scale that up by three orders of magnitude, and consider this. The Uninvolved perspective was looking around between relative moments to see more stars die. Truly coordinated interstellar operations are effectively impossible to coordinate that closely due to how First Secret drives work.
So when you look at those moments, consider how the phrase "time is relative" isn't just about relativity. It's about what time means to the one experiencing it.