I'm not saying Sol Invictus's hands are clean, but rather that fingering him as the source and sole root of the problem is kinda badly misinterpreting a lot of stuff. While implicitly positing a solution that really isn't much of a solution at all 'cause it's hard to see how any other immediate option wouldn't be just-as-bad-if-not-worse. And in general not being especially sympathetic to what no-shit is a pretty fucked up situation the guy's in.
I will say that I agree with this part, but my ultimate outcome for "who is the Sun?" is that he's... well, he's what people I know can end up becoming. Being compassionate, being moral, being concerned with others, all of that carries pain and stress and pressure, because you have to wrestle with serious questions and try to figure out how best to help people and suppress your own personal desires for the good of others.
Sol Invictus didn't want to be the guy who's always holding the bag when things go wrong, but the Solars failed and the Sidereals failed and the mortals failed and goddamn everybody failed, and kept failing, and fucking hell why does he have to be the only one who can save the world, why does he have to pull Creation's ass out of the fire every. Single.
Fucking. Time?
Eventually, it gets to you. Eventually, the strain of trying to be a good person, of carrying the world on your shoulders, becomes too much to bear. You have to set it down, just for a minute. Except your own compassion won't let you, because while you're sitting around you can hear a million children being slaughtered by some genocidal dictator in the East, and see a good man's betrayal being plotted in the North, and it never stops it never fucking stops, the world is just an endless chain of horrible things getting ready to happen and there's no way to make it sit down and shut up and give you five minutes' peace.
So you learn to strangle that altruistic voice inside you, choke it out so you can rest, just for a little bit. You feel numb and empty and you know you'll feel horrible about it later, but you'll feel horrible no matter what you do, so you'll take your peace where you can get it. Every time, it gets a little easier. Every time, the world gets worse and the workload piles up even more.
By degrees, you start suppressing your humanity more and more, seeing your own conscience as an enemy to be suppressed and opposed and pushed out of sight. Haven't you done enough? Can't the rest of the world fend for itself without you there to play nursemaid? It's not so much to ask, is it?
You deserve this.
And then one day, you wake up and realize there's nothing left but silence where your soul used to be; the voice of humanity has finally, finally died, and you realize what you've done, what you'll continue to do, what can never undo. Drown the pain in whatever you can, block everything out and try to forget, let everything slide into decay and entropy and just do whatever you have to to get through the day. You're miserable and broken and empty and hollow and damned, a corpse that walks, rotting from the inside and there's nothing you can do, nothing you can do, nothing you can do. Say it until you believe it. Say it until you don't feel like a liar. Say it until the words break down and all you hear is white noise, as empty as your world has become.
You deserve this.
The Sol Invictus that was, the figure of hope and power, only exists within the souls of the Lawbringers. He failed, but they can succeed. They are the future, and he is the past.