Why do I get the feeling that Rhaast is the most harmless of all the Darkin?

Not sure why, you might call it a random intuition or something, but let's take a step back for a minute and compare him with Varus and Aatrox: don't those two appear a lot more dangerous? Yes sure Rhaast is loud, and loves playing the "muahahaha I'm so evil kill them all"-part, but then again the ego of an edgy teenager is enough to keep him in line and his weapon is the most clunky thing you could ever use in close combat. Even when you have the necessary strenght to make it lethal, a farming tool is a farming tool, and not a weapon. Like I could understand if a Darkin was banished into a random object if they were a mage or something, but Rhaast is obviously more on the warrior-side of things.

And we also know that all the Darkin are the surviving ascendants of the ancient shuriman empire, those that were purged by an targonian intervention and sealed. The only ones who survived this were Renekton, who like Xerath was sealed away and probably thought dead by most, and Nasus who at that point had become a hermit driven almost insane by grief. Azir was pretty much stuck in the RAM-Storage of the sun-disk as his ascension had an unscheduled shutdown with a server downtime that spanned several thousand years.

So at some point those Darkin were one big group or rather family. And if you look at it from that perspective, doesn't Rhaast appear a lot more, I don't know, "childish" compred to the others? He tries so desperatly hard to look evil and murderous to the point of even picking a weapon because it looks evil and cool over its actrual fighting capabilities. Meanwhile, the others don't "try" to look like they are about to end the world. It's pretty much their default state.

By my theory human Rhaast was never really a scholar or a warrior, but someone who through countless accidents became a shuriman hero, ascended, and then tried to find his place in a group that he knew he was definetly not qualified for to be a part of - and to compensate for that he tried so, so hard, to the point of it actually backfiring on him.

My guess is, that Rhaast is the "most human" Darkin there is, because he was also one of the "weakest", and most human ascendants - and that opens an interesting plot line for Kayn and Rhaast: when the evil demon weapon now suddenly the one with more humanity than the one wielding it. Because let's be real, Kayn is one giant murderous mess psychologically speaking. Child-soldiers turned assassins are not exactly known for their mental stability OR their exceptional moral compas - and I think we can also blame Zed's possibly nonexistent parenting skills on that one.

The Darkin will "turn good" at some point, or at least they will stop trying to end the world, but one of them has to turn first. The typical (lame) storyline for this would be to have the love of Kal and Valmar convince Varus to let go of his harted, but who wants something as cheesy as that anyways? But if we set the Kayn/Rhaast dynamic at the core of all this, don't things turn a lot more interesting?

Tl,dr: You thought Kayn was the edgy one trying too hard, but it was Rhaast all along!