When trying to drop the SDE user, you are getting the error:

ORA-21700: object does not exist or is marked for delete

A corruption is introduced in your ArcSDE Geodatabase instance:

1. Try to drop user SDE using drop user cascade command: DROP USER SDE CASCADE;
This will generate error if there are objects created using SDE.ST_GEOMETRY because of dependencies

2. Try to drop the tables in other schemas using the SDE.ST_GEOMETRY type
Get also error ORA-21700: object does not exist or is marked for delete

3. Back to SDE and try to drop the SDE.ST_GEOMETRY type gives error
ORA-21700: object does not exist or is marked for delete 

The Cause of this issue:

Dictionary gets corrupted when tables using a specific object type (SDE.ST_GEOMETRY) are tried to be dropped after the type was tried to get dropped 

The Solution:

the objects cannot be dropped after this type of corruption is introduced

The objects can be left there and create a new database for this cartridge to be reinstalled or create a new database and do full exp/imp

In order to avoid this problem the objects using the SDE.ST_GEOMETRY type have to be dropped before dropping the SDE schema. Also the drop user SDE cascade should not be used.

ESRI Reference Link: http://support.esri.com/en/knowledgebase/techarticles/detail/34483

Oracle Metal ink reference document number 9876