Bizarre to imply hours flown in anyway is responsible for 'losing it', if anything it's the opposite.
I was down route as a fairly junior LH FO, while having a drink with the skip he totally lost it and got furious about an internal political disagreement. He'd been around for decades and seemed to view things as not as good as they used to be and pilots no longer having respect internally. Not saying he didn't have a point but this anger spilled over to his attitude with the rest of the crew, he was very very angry.
Now I'm not implying he would go on to commit mass murder but more to make the point that 600 hours or seniority is largely irrelevant as an indication of anger towards the airline or the world in general.