Well, technically speaking, the shooter’s manifesto was dual pillared, with one pillar being anti-immigrant ethno nationalism, and the other being environmental sustainability. The connection between the two being population control.
This is not an endorsement of his actions. I’ve read the whole manifesto, and I find it weird, to say the least. Perhaps abhorrent may be a better word, given what it led him to do. But after reading the document, environmentalism was one of two primary drivers of it. It would be disingenuous to represent otherwise.