They like killing/enslaving them, yes. One of the first Tau/Eldar interactions was between DEldar and Tau who were being attacked by Tyranids and it's frankly hilarious.
So basically this Tau world, Vigos, was under attack and getting it's ass handed to it by Tyranids. Reinforcements from Rubikon were several weeks away and would probably not arrive in time. Then a strange chap calling himself Urien Rakarth appears and offers to help in exchange for a sample of each Tau caste to learn more about them. 77 of each non-Ethereal caste and 7 Ethereals as I recall. The Tau agree to the 77x4 but not the Ethereals.
Rakarth and his coven then unleash an army of Wracks, Grotesques and Pain Engines on the Tyranids and wreck face in tandem with the Tau, then take their payment away in their spiky horrible ships. The Tau are feeling kind skeezy about this cause the flesh monsters are creepy, but they're used to working with races they find grisly, like the Kroot. Plus they have no idea Wracks and Grotesques are actually made so they decide it's worth working with Rakarth in future. They prep for a counter attack, but they notice after the fighting that a lot of the new Grotesques fielded had familiar blue-grey skin and realise what happened to their sacrifices.
Immediately after they realise this Rakarth calls them up and ask for either his 7 Ethereals or 7077 other Tau. Now kinda mad at him the Tau try to attack his ship with their recently arrived reinforcements, only to find his ship is just a hologram and fake sensor signatures. Then they get a distress call from the planet the reinforcements came from. Urien Rakarth is attacking it now it's garrison fleet is away, along with Vect himself and a healthy smattering of kabals and covens joining in. By the time the Tau got back there to try and help the place was a charnel house of corpses devoid of life, most of the people having been spirited away to Commorragh and the others arranged into an insulting menagerie of meat.
To make matters worse, the Tau were livid and located an Eldar homeworld to retaliate. The problem was, these were Exodites, basically space Amish. While they put up a stubborn resistance at first, the Tau decided to just exterminatus the whole planet and be done with it. Not understanding that Exodite Eldar and Commoragh Eldar are two different factions, the Tau soon became acquainted with another faction of Eldar: Craftworld Iyanden Eldar, the most warlike and xenocidal of the Craftworld Eldar. The Iyanden Eldar were not amused that the Exodites had been slaughtered in an unprovoked act of senseless murder and set correcting the Tau regarding the differences between Commoragh Eldar and Exodite Eldar. Which is to say the Tau attacked the Craftworld Eldar and found that the Eldar forces were more mobile than first expected, and their weapons considerably more powerful than first anticipated. After getting soundly thrashed the Tau called it a draw, with some adorable lore along the way of the Tau finding damaged wraithguards & Earth caste Tau desperately trying to figure out how the Eldar made such advanced battlesuits. The battle ended with the Tau trying to form an alliance with the Iyanden Eldar, who rejected it, but nevertheless agreed to make the Dark Eldar pay for all the unnecessarily lost Eldar lives, the Tau concurred for their own losses.
Also as a related note, the Tau reclassifying Space Marines from a sentient caste of humans to 'living weapons' (literally calling them 'engineered humans') akin to the tyranids is both amusing and grimdark. The Tau-Space Marine interactions are one of the few nice times you get to see spehss mahrines from a new light. Stuff like Tau commanders dismissing reports of space marine strategy as implausible, as no force could insert their troops directly into Tau formations without falling to their AA fire, or arriving without the required heavy weapons needed to deal with Tau armour. Their first experiences with drop pods, space marines, terminators & dreadnoughts landing on top of their formations were quick learning experiences.
The Tau get a few other helpful experiences with space marines. Like happy fun time with the deathwatch infiltrating their world with penal guardsmen to assassinate ethereals. Or regular space marines deep striking on top of ethereals for the same reason. In one colourful instance with an ultramarine, a water caste diplomat successfully manages to guilt trip the ultramarine - very bravely & shrewdly talking the ultramarine down, mere moments before it was about to shoot the diplomat. The diplomat argues in fluent gothic that it knew enough about space marines & their fondness of heraldry to notice that they were knights, and how would their family & their people react if they could see him now, killing diplomats who were no threat to him? The space marine responds by saying he was just conserving ammunition and steps on the diplomat instead, but inwardly is rattled by the ordeal. But to the observing Tau, the voice broadcasted through its vox speakers was emotionless & hostile, pretty much confirming their suspicion that space marines couldn't be integrated.
Their final conclusion comes when they capture a raven guard space marine. Using mind worms (exactly like Alpha Centauri's mind worms, pretty gud) and Earth caste enhancement machines, they try advanced interrogation methods where traditional ones failed. Half the worms in the mind worm boil die and a few of the machines burn out just breaking through the raven guard's mental defences to get deeper into his mind. They find his childhood memories of living in a Hive World, and the Tau are largely disappointed with how completely and utterly shit it looks, tiny cubicles separated by tattered cloth to form makeshift rooms, all in the thick of industrial smoke and danger. The mind worms follow this memory of the boy in the hive looking up at the moon, and unexpectedly find themselves looking at a battlefield full of slaughtered humans, with the raven guard expressing satisfaction at the scene. The mind worms are confused until they understand the raven guard is telling them the dead bodies were traitors, that this is the fate of all traitors - at this point the mind worms are a bit unnerved because they realise the raven guard is resisting them, something they hadn't known was possible before.
Burning through their mind worm colony, boosting their enhancement machines to 87%, they bring in a secondary mind worm boil to reinforce the first. The mind worms tell the Earth caste scientists that they're going to die, or they're going to break the Space Marine's mind before they can break its will. The space marine concurs, as while they do extract the memory of the child looking up at space marines descending from the sky (presumably, the day he ended up getting recruited), the raven guard spams them with a hundred memories of burning worlds, exploding their enhancement machines and mind worms. The Tau conclude that even with extraordinary means, the space marines are a lost cause and should be killed immediately to neutralise them / demoralize neighbouring guard units.