He's a phoney and not insane. That was an act. There was some thought at the time that he was a member of a white nationalist org in Florida, but the leader of that org backed off of this prior claim. Evidence is inconclusive.
Nikalas was a MAGA guy though and an extremely hateful racist to be sure. Multiple photos of him sporting the beautiful red trucker hat. His instagram avatar was MAGA-themed. Huge racist. Snopes is your friend if you like the truth. From instagram private chat:
https://www.snopes.com/fact-check/nikolas-cruz-white-supremacist-group/
In one part of the group chat, Cruz wrote that he hated, “jews, ni**ers, immigrants.”
He talked about killing Mexicans, keeping black people in chains and cutting their necks.
There are hundreds of racist messages, racist memes and racist Instagram videos posted in the group.
One member even joked about Cruz’s particular venomousness, saying that although he hated black people, too, he didn’t “to a point I wanna kill the (sic) like nick.”
Cruz said he hated black people simply because they were black; Cruz hated Jews because he believed they wanted to destroy the world.
After one member expressed hatred for gay people, Cruz agreed, saying, “Shoot them in the back of head.”
White women drew Cruz’s hatred as well, specifically those in interracial relationships, whom he referred to repeatedly as traitors.