The "immediate threat to human life" was not identified specifically; the only post meeting that description was taken down by moderators in less than an hour (which compares favourably with many social media sites' response times to similar incidents) and was from a new account that hadn't posted anything else, so hardly representative of the site's culture.
While I don't know the specifics behind your other link, like most articles about it it's using second- and third-hand claims; given there was an orchestrated smear campaign against the site, I'm sceptical. It was certainly convenient that Kiwi Farms was removed from the Internet Archive just as articles like this were being published, making it impossible to verify or disprove any of their allegations. If you're worrying about sources (which you are absolutely right to!), I'd start there; do the posts that article is talking about actually exist?
It's about as impossible as proving what "caused" someone to die after they got run over by a train. They could have had a fatal aneurysm in the moments before getting run over. Or perhaps it was the train that killed them. A perplexing game of probability indeed!
One of the three suicides seems to have not happened at all (would have been reported to the US consul in Japan), two others were attributed to posts that did not comply to KF moderation standards, were created by freshly registered users and were removed much faster than similar stuff typically remains visible on Twitter of Facebook.
Thought experiment: if KF founder committed suicide, would you demand deplatforming MotherJones?
A less generous interpretation is that they coordinate harassment campaigns against transgender people and have caused at least three suicides.