I've never read or posted to The_Donald before although I was aware of its existence, so I read your post with a neutral mindset and followed the links to verify your claims. Because if true then I'd agree with you on its closure.

Many of the posts are quite nasty, for sure. But I feel rather deceived. Many of the cited links don't support your claims about them. I checked them all, here are the summaries here for others.

advocated for violence against leftists [1]

This post advocates deportation of low-IQ immigrants. Many countries impose proxies for IQ on immigrants, e.g. degree requirements. I've had to pass such requirements myself. For this claim to be correct any border enforcement must be considered "advocacy of violence", which given the widespread existence of borders would be in my view a misleading claim. None of the replies advocate for violence either.

calling for civil war in response to impeachement [2]

This post doesn't "call" for civil war but predicts that it would be a likely outcome of Trump being replaced by Nancy Pelosi i.e. someone of entirely opposite views, in case of impeachment. If this post is considered unacceptable speech then it's hard to see how anyone could say "action X would lead to civil war" without it being equally unacceptable, which would effectively ban all discussion of civil war including by academics.

calling for the murder of those who disagree with them [3]

This post says "Whats the penalty for treason? Death?" and the replies give a variety of other possible outcomes like CNN jobs, book deals etc.

The US does have a history of executing people for treason. Lawful executions aren't legally considered murder. This post is thus a rhetorical question about constitutional fact. Obviously whether the act being discussed really is treason or not is a different matter, I didn't bother checking that, but it's not the same as saying "let's go murder this person" because the call is for the state to take action, not individuals.

general gay and islamic hatred [4]

This one is an image macro titled, "I present to you, the anti christ". The image is a cartoon of what appears to be a gay man wearing a turban and smoking a cigarette. It does indeed seek to denigrate gays and muslims, I guess, if you accept that depicting a gay muslim is inherently "hatred". But isn't that a contradiction in terms? Isn't that only considered controversial because Islam is famously intolerant of gays? The combination of these two things really shouldn't be considered a problem, according to western values, and presumably that was the point.

kept a discord with doxxes of leftist in their sidebar [5]

The leftists in question signed a public petition, and petitions aren't signed by anonymous people. Still, I agree this breaks Reddit's rules on doxxing as addresses are more than names, and this is by far the strongest claim to bad behaviour so far.

calling for political opponent's murder [6]

This one calls for Hillary Clinton to be imprisoned, not murdered. A small minority of comments talk about treason, but again when the state executes someone this isn't called murder, which is a legal term with its own meaning.

rounding up muslims into concentration camps [7]

This one indeed is bad, it describes China rounding up muslims in concentration camps and then says that whilst they "aren't applauding China" maybe the EU and Canada can learn something from it. For me that'd cross the line.

The top voted comment and thread is describing muslims as feral animals, barbarians etc. The others are suggesting that China has legitimate reasons for it due to Islamic terrorism.

extermination of migrants [8]

This one says, "ICE isn't enough. The Federal government will need to implement martial law to eradicate the aliens and corrupt politicians"

This one seems ambiguous. Grammatically it can be read as either ICE isn't enough to fulfil its mission of "eradicating aliens" (deporting them would be getting rid of them), which doesn't involve "exterminating" people let alone all migrants as they only care about illegal immigrants. Or it can be read as the difference between ICE and martial law is the level of killing involved. Given the Hacker News rule of the most generous reading possible this one would go the first way - not a call for "extermination of migrants" or indeed politicians but rather tougher enforcement of the existing rules.

The replies are all discussing electoral demographics and don't make any statements about killing one way or another.

The final three you didn't summarise. Oddly they seem like the strongest to build your case. They are anti-semitic comments about how "jews control our media" etc, a comment about "humvees with mounted machine guns" in response to a video of quite mild clashes between migrants and police, and a comment revelling and glorifying the impaling of someone who attempted to clime a border fence and was killed by the fence's protection mechanisms.

Clearly there's some nasty content there the moderators failed to crack down on. But I've seen some very nasty posts on all kinds of subreddits over the years. To prove T_D was worse than others would require some sort of statistical analysis, which would itself be likely to be biased (obviously a lot of posters on Reddit think nothing of calling people they disagree with Nazis... and we know what happened to them)