Disclaimer: I don't know shit about this guy. I can't speak about his particular case or personal motivations and circumstances. With this in mind, let me
instead answer the more generic aspect of your question.
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Some illnesses are hard. But even so, they are temporarily bad. You suffer some time, get some treatment or procedure and then it's -mostly- over. You may lose something in the process, but generally you can go on with your life after it.
Sometimes there are permanent effects. Like maybe you lose an eye, or maybe you have to be medicated for the rest of your life, or have to keep a special diet or something. Or you may get some permanent discomfort. But again, even with this, you can generally continue otherwise "well enough".
Some other illnesses are hard, and lethal. You may suffer, go through some process -or not- and then die. These are hard to endure because, well, you know you're dying. But then again, it happens fairly quickly.
Some other illnesses can be hard and recurring. Like a cancer or lymphoma. The treatment is hard and exhausting. But you go through it and it either works and you get some additional years to live fairly ok, or it doesn't work and you rapidly go away. Then they return and you repeat the loop. But again, you mostly live mostly ok some years and then get hit again, go through treatment and then the fork of either having some years more or dying "quickly".
Some are constantly hard. In the sense that they don't kill you but make you live with constant and relentless suffering. The psychological impact this has is hard to overestimate and while you don't die, it can be said that they take you life because they change it so completely that you have almost nothing else but fighting constantly against the pain and suffering.
When you are the subject yourself of such a situation, the effect can be devastating. Different people react differently, of course, but there's always some psychological damage. This can sometimes produce its own neurological illnesses that pile on top of it all.
But you may not be the subject of such an illness and still suffer the psychological impact. If you're a person that cares and someone close to you falls into that situation it's very easy to be affected. You won't experience it first-hand but you will see a person you love suffering every single day. It's worse when it also happens at night. Because then they will suffer and they will be significantly impacting their own health through sleeping badly or not sleeping at all. And if you're close enough to be there, chances are you will also sleep badly and affect your own health too.
Sometimes the situation means that you really can't do shit about it. Sure, you can be there, give them support, your love, your care, etc. And that is indeed a lot. But it has no particular effect on the illness itself so it can easily feel worthless, pointless, useless. The psychological effect of all this is both subtle, in the sense that you may not even be aware of it, and fairly impactful, producing changes in your personality and mental health.
Sometimes, other circumstances work together with the illness for the caring person to have to make big sacrifices. Like maybe quitting their job or career, or their own family or friends, or whatever. Sometimes they end up developing their own maladies because of this situation -or sometimes apparently because of it-.
And so, a person who is generally healthy gets to see someone they care for suffer continuously every moment of their life, and they are forced to renounce big parts of their own life to care for them, and then they are impacted with subtle but deep psychological problems. The description of "it breaks your heart" is quite appropriate because you may be giving all your love and effort while simultaneously feeling completely useless, and end up inflicting hard damage on yourself.
Different people will react differently to all this. But it's hard to predict how any of us would react until you've actually gone through it. Sure, you can say "I'd seek help" or "I'd try to stay positive" or even "I'd certainly go crazy", but the truth is you don't actually know.
For some people it's not uncommon to react by looking for an "ultimate cause", something they can attribute all the problems to. It can be a generic cause like "life sucks" and they may end up bitter against life in general. Some turn religious. Some do the opposite. Or it may be that they find fault on something they did or didn't and so they end up blaming themselves, with various outcomes. It may also be that they blame another close person, a parent, a sibling, and it's not uncommon to see families split over such illnesses. Sometimes they may find a cause in "the system", in a negligent doctor, in an "uncaring" administration, in causes with different degrees of distance and specificity.
The problem has many aspects contributing to it and each person, again, will react differently. But sometimes it just happens that this one person under the accumulated effects of suffering, of seeing someone not die but live in agony every moment, finds this "ultimate cause" personified on some organization or some one specific person who can maybe -in reality or in their reality- have caused that pain or have profited from it or whatever, through their actions or inactions. With enough persistence, it's not hard for all of that to transform into rage or hate and, sometimes, produce the effect you see here.
I'm not saying this reaction is inevitable, or logical, or forgivable, or anything. That's up to you to think. Just that it's not impossible to understand and that there may be circumstances that push people... that crush people and them push them into tragic actions.
Finally, yes, I agree with the conclusion that it's terribly sad. In a lot of ways.