TW: Heavy talk about death
I think the reason why Kai and Cole had such a radical emotional change after March of the Oni was because Lloyd died
We’ve had plenty of characters die in Ninjago, and it always had a horrific affect on the ninja emotionally. Despite all their adventures and godlike powers they’re still older teenagers and young adults- of course when their sibling or lover dies in the fight they’re gonna want to avoid the fight itself
I think it was different for Lloyd tho- Lloyd despite losing the golden power is still the most powerful ninja. He’s not meant to lose, out of all of the ninja he’s the one who’s least likely to die and I think they know that.
So when he died facing down the Oni, even briefly, that would make the ninja doubtful and even afraid of their own mortality-
Kai’s hotheadedness and desire for the fury of battle dies when he see’s his little brother unresponsive on the floor. If he can die, so can Kai
Coles stubborn facade and die hard attitude gets a reality check when the kid who defeated the incarnate of evil lies dead in front of him
It would just be a reality check for them. Both have had to look death in the eyes many times before, but experiencing death is something that they themselves wouldn’t often think about.
Just a theory