It's not speech. You have a gun that fires when you say a word. You having a setup that fires the gun when you say a word instead of pulling a trigger doesn't mean you are utilizing "speech" in any meaningful sense of the word, other than that the gun responds to a sound.

There is no gun involved. I say the words, and the person dies. Should the government restrict my speech?

Again, we don't live in a fairyland, we live in reality. A gun is real, your magic is not. If you had a gun that fired based on a sound, the fact that a sound is used does not matter. It is not "speech". Noise is not protected speech.

Likewise, if you had the magical ability to kill a random person with your magical sounds, you are not practicing "speech", you are using a magical technique in your make believe land to kill random people.

Yes, for some definition of "speech which causes deaths". This definition should include calls to violence but exclude, for example, vaccine skepticism.

There's a big ol' messy line between a powerful entity saying "I would like you not to do that" and "I'm going to stop you from doing that".

We don't want to be in some world where the gov can't ask people to stop being/hosting vaccine deniers.