Angels dont cease to exist when “killed”, they,like a demon, have their essence dispersed and over time it like coalesces and the the spire vibrations reform the angel,and demons reform in the burning hells. At least its been my understanding since D3. This is why it was called the Eternal War,as each side never lost anyone truly,as they eventually reform back at their respective realms. Thats what happened to Tyrael. At start of D3 we see him reform in heaven after his sacrifice on mt arreat. It took him years to gather is essence enough for spire to pick up or something, his vibrations and brought him back or reformed him or w.e.