You want to create/delete entities from a system? There is good news for you. You can just use Entities to do that. It implements SystemData so just put it in your SystemData tuple.

Don't confuse specs::Entities with specs::EntitiesRes. While the latter one is the actual resource, the other one is a type definition for Fetch<entity::Entities>.

Please note that you may never write to these Entities, so only use Fetch. Even though it's immutable, you can atomically create and delete entities with it. Just use the .create() and .delete() methods, respectively. After dynamic entity creation / deletion, a call World::maintain is necessary in order to make the changes persistent and delete associated components.

You cannot, however, easily build an entity with associated components. For that, you have to write to these component storages and insert a component for your entity.