INSERT [TableName] SELECT * FROM [TableName]
That SPID is already killed and you will have to wait for the rollback process to finish. The estimated time is usually wildly inaccurate: don't worry about it. It will probably take from 1 to 2 times the execution time of the INSERT until the stage when it was killed.

If this happened in production, the answer would not be any different: wait for the rollback process to end. The database would not be consistent if the rollback operation was forcibly closed.