2. The very assumption of it being impossible to coexist with other entities capable of reason is without merit. The Necron, Tyrannids, and Orks are good examples of why 'only war' exists in the setting is that they cannot be reasoned with at all.
It's actually... ugh...First of all, at least in more recent fluff, Necrons can totally be reasoned with and they actually have personalities, but basically they all are so huge plasteel dicks they just kill people for kicks. Their whole existence is actually based on one of oldest (we're talking like millions of years before humanity) stupid and bad decision in Warhammer universe, when they decided that they're jealous of Old Ones and Eldar and so on, since Necrons back then were basically edgy shitlords crying on their own planet getting slowly killed by the radiation. So they took those C'Tan guys, put them into huge bodies of living metal because WHY NOT, got their asses handed to them and started worshipping said C'Tan because WHY NOT, and then themselves got turned into bodies of living metal because WHY NOT, and then they started the whole War in Heaven because WHY NOT, which basically was the sole reason for all the retarded Chaos fuckery in the whole universe, ending with Old Ones also creating Orks to combat Necrons and not giving them a killswitch, all of this because WHY NOT and WHAT AT WORST COULD HAPPEN. Not to mention that mid-War in Heaven C'Tan started to fight each other despite losing heavily to Old Ones, Eldar and Orks because WHY NOT.
And Tyranids appeared out of nowhere because WHY NOT.
Like, okay, most of this stuff is actually pretty solid, which is why I don't understand what pieces you claim are worst decisions and lack of reason for anything, when the stuff you call being actually okay is exactly what you said, worst decisions for no reason at all. Like, point me at something. What seems without reason to you? Horus Heresy, the most explained on event in whole Wh40k universe?
Now you see, that is something I like. Orks love to fight, and killing an opponent is by far the highest expression of that mentality.
Eh, fighting with you and actually letting you live through it (they aren't going to make it easy though) is the highest expression, since if you die to them, you're worse than them in the end. Ghazghkull Mag Uruk Thraka and Yarrick, go figure, though Ghazghkull itself is pretty special, as it's one of few Orks that can speak High Gothic flawlessly.