As a leftist, I appreciate the good faith effort, but as you probably expect, I don't agree with your interpretation.
So to me, the first issue is that "the left" does not have a universal worldview. You have people on the left who have a very materialist view and generally reject concepts like good and evil, you have idealists who focus on autonomy/personal freedom and you have moralists who have more religious and moralist roots and more connected to the idea of "protecting the weak". The last group is probably the group that comes closest to your interpretation. I personally fit more into the second and first group, but obviously at the end of the day, I can only speak for myself.
I would also make a distinction between liberals and the left. Liberals have adopted leftist talking points where they think it helps them, but they don't really believe in them.
Undeserved suffering is evil. Undeserved suffering is the only evil there is.
Here I already completely disagree. Suffering isn't evil, suffering isn't even bad, suffering is an fundamental part of the human condition, arguably a fundamental part of life itself, it's kinda beyond being strictly good and evil. A human who has never suffered is unimaginable, a world without suffering sounds like a distopia to me.
Deserved suffering is fine, so you can torture Nazis - they deserve it.
99% of leftists are against torturing anyone, not because it causes suffering, but because it violates people's autonomy. Many leftists tolerate violence against fascists/nazis, but they see it as self-defense.
Since undeserved suffering is the ultimate evil and the only evil there is, we must rearrange government and society to eliminate all unwanted suffering.
If you replace "suffering" with "oppression", you get closer to the real picture. So again, this does not apply to all leftists, but many leftists are focused on "oppression" (which is essentially restricting people's autonomy/personal freedom).
Abortion is seen as a heroic act
Abortion is not seen as a heroic act, it's seen as a morally grey/complicated act, which is why most leftists want this complicated choice to be made by the person most directly affected by the moral dilemma instead of having the state forcing it's morality onto people by force..
Even if it is murder they don't care.
Abortion is seen as different from murder because a fetus is not considered to be an independent person. A human fetus is indeed human, it is indeed alive, but a person only becomes a person once it is born and is able to exist as a seperate entity. That's why we celebrate birthday's as the start of a person's life, that's why we have a birth certificate to document a person's life and give them a name. It's impossible for a fetus to be removed from it's mother without dying, which means a fetus cannot posess any form of autonomy, so in function, it is part of the mother's body.
This doesn't mean that abortion is morally clear or unproblematic, but leftists do believe that the state should not make that decision for the mother.
They will deny this, but the rampant nihilism on the left all but proves my point.
Nihilists do exist and they do indeed tend to be lefists. But nihilism doesn't argue that "existance is evil", nihilism rejects labels such as "good, evil, rightous, ..." and in a radical form rejects a "higher purpose/higher meaning" in a religious and/or spirital sense. So no, the left or even leftist nihilists don't believe that existence is evil.
Need a carbon tax to save the planet but it will kill a billion people?
This argument does not seem to be written in good faith. People who support a carbon tax don't do so "to save the planet" and they certainly don't do so to "kill a billion people".. Also, carbon tax is not a leftist idea, it's is supported by a wide part of the population accross the political spectrum. 73 of all Americans support a carbon tax, this includes more than half of Republicans.
https://www.pewresearch.org/science/ps_20-06-11_climate_featured/
And the idea behind carbon tax is a pretty moderate one, it has nothing to do with "saving the planet", it's about who pays the cost that is generated by emissing carbon. Without a carbon tax, the cost is covered by everyone, irregardless who produced the carbon emissions. This means that even if you don't produce a lot of carbon, you have to cover the cost.
Most people think this is obviously unfair and believe that the cost generated by carbon emissions should be paid proportionally by those who created the carbon emissions and generated the cost in the first place.
Just imagine you are going out to eat with a couple of people. You just take a water and a salad, most just take an average meal and some just take the most expensive meal on the menu. Do you believe it's fair if the bill is split by everyone equally so that everyone pays the same? Most would say no because it means that those who ordered cheap food have to pay more than they ordered and the people who ordered expensive food have to pay less than they ordered.
I'm not saying carbon taxes are the solution for everything or that they could not have negative impacts, especially depending on how it is put into practice, but implying that "people who support carbon taxes" do so because they think people existing is bad and they want to murder everyone does not seem at all like an attempt to make a good faith argument..
You as an individual human being maybe don't. But online, that's irrelevant. People don't see you as a human individual, for better or worse, people see you as part of a group/community. And by most people on lemmy, the community on this server is seen as rude and unpleasant.
And not only "the left" places individuals into groups, everyone does that.. You do it in this post when you say "They argue in bad faith, they are rude and generally unpleasant.".
It is possible when people actually try, but in order to reach a point where people want to try to understand the other side, you need a tiny portion of faith that the other side is willing to engage in good faith and make a step towards the other side to signal that they are actually interested in good faith.
One big reason (from the leftist perspective) why many online leftists have completely abandoned any hope to engage in good faith with right wingers is because online right wingers seem to go out of their way to be obnoxious, provocative, unapologetic, uncompromising and disrespectful in every possible way towards "the left" and seem to have the main objective to "trigger the left". And even when they themselves don't engage in this way, they tend to protect and defend those actions. At the same time, they also claim that "the left does not want a good faith argument" or "we just want rational debate" or "we just exercising our free speech".
As a leftie who has tried to engage in good faith with right wingers online in various forums, I have experienced a lot of hypocricy where people will say "this is a free speech zone, all views are welcome", but after a while, the admin would just have some bullshit excuse to ban me for simply voicing my opinion.
But as I mentioned, there are many reasons, it's not just all "the right's fault". Online people of all sides, instead of arguing and discussing with people they disagree with, tend to just kinda argue with themselves by essentially imagining the position of the opponents and then arguing against them.
The obvious problem with that is that due to the lack of interaction and the obvious bias, the view that one side has of the other does not represent reality or at least differs a lot from the view one side has of itself.