Sometimes we toy with a headcanon where the 40k galaxy is actually a super-benign place where most sapients rarely resort to violence, sort of like in Alan Dean Foster's The Damned trilogy. And all the playable factions are those rare creatures that relish it, or see it as a legitimate means to an end.
Here's our take on each of the 40k factions (except chaos), based on the actual fluff when viewed from the right angle.
Eldar (space elves)
Most of Eldar society abhors violence, and seems to be basically incapable of it. They have highly-ritualized warrior "paths," which include things like the use of psychic helmets to focus their thoughts and suppress their remorse. The leaders of their "aspect warriors" are universally pitied, because they are the unfortunate ones who became subsumed in their psychically-imprinted war personalities.
Orks (space orcs)
40k's designated "comic relief" characters, based on British football hooligans. Their society is a literal joke, where the punch line is always "more dakka."
Necrons (space skeletons)
Terminator robots, created by a society of Great Old Ones to fight the Eldar. Now entombed on thousands of worlds across the galaxy, only a few of which are awakening due to chance events. The Necron "overlords" on these tomb worlds have extremely bad mental illness due to programming degradation, and command the other space skeletons who lack individuality.
Tau (space communists)
Uplifted by the Eldar, who secretly engineered "Ethereal" Tau with mind-control pheromones. Their cooperative natures made them extremely pliable, to the point where the Eldar could create separate "castes" (breeds) of Tau, including one that was made just for ground combat.
Even the Fire Warriors, though, are physically weak and unimposing, and rare is the individual with enough
determination courage to stand up to the horrors of war without an Ethereal compelling them. Most of the ones that do have sufficient backbone are given the "hero's mantle" (battlesuit), an anime mecha that multiplies their ability in combat.
Tau alien auxiliaries
Dozens of species have supposedly been integrated into the Tau Empire, but the only ones we ever see on the (planetside) battlefield with them are humans, Kroot, and Vespid. Kroot are highly-athletic, natural carnivores, and the ones who fight alongside the Tau are mercenaries who see it as an extreme sport and a chance to vary their diets. Their loosely-knit squads break easily when challenged, only to reform after the battle once it's safe.
Meanwhile, Vespid are insects, that it's strongly implied the Tau learned to mind-control using Ethereal-style pheromones. Remove the (pheromone-emitting?) Strain Leader from combat, and the rest of the Vespid squad scatters.
Who freaking knows, man. If Tyranids have an alignment, it's like, "Chaotic Hungry." Plus, they aren't strictly from the 40k galaxy.
Called gue'la ("Morbid Ones") by the Tau, humans are highly-social primates, whose identities are so easily subsumed into factions and groups that they lose the ability to empathize with outsiders, who are no longer considered people. The Imperium of Man is the ultimate example of this mentality, as it considers all "xenos" and non-Imperial humans to be hateful monsters.
Much like today's humans, Imperials undergo lifelong conditioning, which teaches them to obey orders and that their individual lives are meaningless. The only things that matter to them are continuing the legacy of the past (embodied by the skulls that Imperials venerate and adorn their buildings with), and killing the hateful outsiders. Their "Angels of Death," the Space Marines, are quasi-mythical beings, who are physically transformed into monsters and practically worshiped by those rare humans who glimpse them.
Without this extreme conditioning, though, or in more benign social environments, humans can be friendly and docile. It helps to avoid inequality, to separate them from factional markers, and to promote empathy by letting them mingle with those they consider "outsiders." The Tau have had great success in so rehabilitating the humans who find themselves in their domain.
So is this just heresy, or are we on to something here?