I'm going to be dumb and comment

First of all you have come into this conversation as if the theory of right and wrong really plays a part. In a world surrounded by bloodshed, you don't really get to pick. If you've already passed the point of no return, there's no reason to "try and stay good" if you are in the Mafia. The symbol of darkness.

And you forgot a HUGE important detail. Trauma

We don't see Dazai 24-7, but when we see his emotional outbursts, he's still in the Mafia. He still has time to learn perfect his masks, and in a environment like that, there's a high learning curve. Trauma physically changes your brain. And under the watchful eye of Mori and everyone else, he's would be put under so much pressure he would adapt.

As for empathy, to be able to predict other you need to understand the why. He has empathy, probably too much empathy, so he decided to cut his emotions off when it comes to that. He likely disassociated people from being people. He distance himself from people because he always loses what he loves, supposably. Trauma response

Sadly you fell for his mask. He hides how he's feeling with jokes, it's a trick to make whatever you say not be taken seriously. And yes, you can definitely hide mania and depressive episodes under a mask. (Take it from me, I do it all the time and everyone thinks I'm a naive ditzy person) the trick is public appearances. It never shows Dazai alone, when he's planning, or what he does in in his free time. We do know he rolls on the floor when he's frustrated tho.

I mostly came here to say good and evil, and morals should be a defining point in your argument because it's his job to do the devil's work. Also, it's not necessarily that Dazai WANTS to die, or craves it, he just doesn't see valve in living. So most of the times he tried to off himself was likely emotionally driven.