The tyranids are an expression of entropy more than they are a species, they’re very nearly a force of nature. They’re not really “good” or “bad” in the same way that gravity isn’t good or bad.

Now the Orkz I think are just fun and good dudes. They don’t really hate anybody in particular, they just LOVE to FIGHT and will keep doing that unless stopped. They will gladly fight amongst themselves if no one else appears to fight with them. They don’t really have grand dreams of conquest beyond swarming over the horizon to fight whatever is on the other side of it. The optimal end state of Ork supremacy isn’t galactic domination, it’s one big WAAGH cloud that rips across the galaxy in a giant loop and takes long enough doing it that the survivors can settle back down and bunker back up before the Boyz come back to town. They don’t even necessarily want to win, they just love to fight, they think it’s the best activity that you can do, and they want to share that with any and everyone.

From anyone else’s perspective this is a horrifying wall of green skinned, brutal cunning monsters that will sprout up out of literal nowhere and reproduce faster than you can print more bullets for them. But from the Orkz perspective they’re basically just asking you to join their football game. They’re fun guys.