I wonder if she apprenticed herself to Maul at some point. Apparently, she was personally assigned by the senate to kill him around the same time as the Battle of Coruscant began in ROTS, but they both became fugitives during Order 66 and the Purge, and we all know that strange and unusual alliances are made in those dark times... plus, the episode clearly shows that the two have some personal history (e.g. the "Lady Tano" remark).