It's an Imageboard, there are many of them. 8chan gained a lot of traffic because of the Gamergate [1] controversy in late 2014 The site has nothing to-do with 4chan in terms of administration.

I can't say anything about how 8chan is involved in the El Paso shooting, since it's been some time I've been active on 8chan, but the Christchurch mosque shootings was announced on 8chan. There are many more of such incidents where Imageboards have played a role - e.g. the killing of a kid by Marcel H. in Herne, Germany was posted and documented on 4chan.

So, it's very possible that they're part of it.

[1] https://en.wikipedia.org/wiki/Gamergate_controversy