Sword logic is a bit tricky to explain, but I'll try my best. You have a purpose. This makes you true. Stopping your purpose makes you false. If you're false, you're not real. And if you're not real, you don't exist. For example, by stopping Oryx's purpose, we define him as false. This is why he didn't actually kill Akka, one of his gods. Akka's purpose, as the Worm of Secrets, was still being done. We also stopped Crota's purpose (wiping out the Light), which defined him as false. This is why he can never return, unlike Xivu Arath or Savathûn. In the mission "Last Rites", where we go to steal the essence of Crota's soul, you may see Ir Yût flying around. We did not stop her purpose by killing her. That is why she is still worthy of existing.