Do conditional 1:1 relationships improve normalization?
Normal forms based on functional dependencies (e.g. BCNF) are defined for relation schemas and relations consisting of values without nulls. As soon as you permit nulls you already have something that is not a normalized relational database.

In your first scenario, if your ADMIN table has (non-nullable) attributes, X,Y,Z, then the dependency {UserID}->{X,Y,Z} is enforced and will be satisfied for every admin user. If you combine the tables and make X,Y,Z nullable then those attributes may be null even for admin users. The dependency {UserID}->{X,Y,Z} is therefore no longer automatically enforced if you combine the tables. Assuming that dependency is important to you then you would have to add extra integrity constraints to validate that each of those attributes has a value for admin users and a null for non-admins. The advantages of BCNF (i.e. having both ADMIN and USER tables) are that the intended business rule {UserID}->{X,Y,Z} is declared as part of your table design, is obvious to anyone inspecting the schema and is guaranteed simply by the presence of keys and non-nullable attributes in your tables.