Mr Christchurch Shooter doesn't have that much power. If he wrapped himself in a rainbow flag, it doesn't mean people should stop using rainbow flags. Ditto him using a thumbs up gesture or an OK handsign. His use or disuse of symbol sin his crime is distasteful, but unimportant.
If he thought he could get people to stop using Islamic symbols by displaying them, he would have been festooned with them.
It might have started as an ironic joke, but if it gets adopted earnestly by actual white supremacists (see the Christchurch shooter using it in court), then in some contexts it can be interpreted as a genuine symbol of white supremacy.