You think deplatforming is going to stop him from killing people? It might make it more likely - he may convince himself that the deplatforming is part of the great conspiracy against normal people, and he has to strike a blow against it. (On the other hand, it makes it harder for him to convince others to join him.)

Don't deplatform him. Investigate him. Get a warrant and find out who's listening to him. Find out what's going on in their private communications - are they making actual plans, or are they just talking?

Deplatforming is about the laziest and least useful possible response. As Fins said, deal with the violence (including plans to commit it).