The problem is the script that is run to kill the Sith on the ritual if the Sith has more than 1000 HP he will not die.
Short term solution is for you to remove the Hardcore mod file k_ai_master.ncs from your override, for just that tomb area, and load a save from before you enter the tomb this should fix that problem, you can then drop that file back in after you have completed the tomb.
stoffe -mkb- had made and posted a fix for that cutscene error, but I'll have to look for it, or someone can post it here for you. ;)
I hope this helps! :D
Edit: Found that post! (
http://www.lucasforums.com/showthread.php?p=1960135#post1960135) Just do what stoffe instructs there. ;)