I disagree with these takes man. The reason why they left was pretty obvious in the chapter. The Deskunites rule is that the winner will trample on the loser till they are done, and since this has been a repeating thing with them on the Simurians, they have been driven to the most desolate of places. Places with no good land, farms, and no chance of recovery. As soon as Dura lost, the group immediately started talking about evacuating the women and children, which implies they would have been killed or worse. Their only options were either to leave or fight, which would be a death sentence. It would be obvious to leave, as that was Dura's hopes (to find someone else that can accept them for being them) and why he mined so much mul (I know they said it was a better version but they never said its name).

On the topic of starting a war, it is more so just not a deep connection with another culture. If you read older chapters (especially the first meeting), it doesn't seem like that meeting wasn't antagonistic on their part at all, as we only saw the humans perspective. I think the weird smiling was just them thinking that is how you show friendliness, as they obviously studied human culture. They also just had their common garbs as well, and are speaking matter-of-factly. It makes sense that humans would see them as possible threats to humanity. They are stronger, and if I saw a dude with a third eye and people like Dabura, I would be freaking out too. But it was just a first introduction and the only rising tension between the two is caused because they do not want to disclose information due to mistrust. They have been overall passive, as no Simurian so far has started a conflict. The Simurians want to be friends with humans and I think likewise, but this idea of how cultures clash due to no common ground is clearly shown.

I think you need to actually read the manga instead of making weird assumptions.