Unfortunately, one of his accounts was already gone by the time I got to it. These three were still up though. I tried to archive as many threads he posted as well, for posterity. As usual, no one had bothered to do much of it before, even though this has been out for at least a day.

Where archive.org archive are missing, it's because it will not allow archives (you first have to confirm that you are 18 and up). But the most prolific account he had did allow archives.

Narrow_Frosting_1539:

archive.today: page 1 - page 2 - page 3 - submissions

AmericanNick7:
archive.org: page 1 - page 2 - page 3 - submissions
archive.today: page 1 - page 2 - page 3 - submissions

AmericanNick:
archive.today: comments - submissions 1 - submissions 2

And here's a thread where he posted now-deleted comments in which he made his intentions clear. Apparently, TwoX moderators did not report it to the authorities, and while another user cautioned against it, he did call it "tempting" and said "you're welcome to try".

Some quite interesting stuff here.

EDIT:

Here's the posting history of the guy who said that killing Kav is "tempting" and "you're welcome to try".

archive.today: comments - submissions
archive.org: comments - submissions