If like me, you did not get it when you murdered a member of your family I suggest the following solution:
1. Change the version of the game to Beta patch_16 on Steam
Right click on the game TWRII >> properties >> Betas tab >> Choose Patch_16 >> a download will start from 1 to 2 GB.
2. Open the game, start a big campaign with any factions (I chose Syracuse). Recruit a new general from your faction (If you wish you can spend several turns before recruiting him). Make a manual save of the game and skip your turn.
3.Go to your family, kill the previously recruited general. (normally you will not have achievement yet)
Load the savegame you just made just before your turn and you'll get it!
It worked for me, I hope it will work for you too.
Happy hunting