It is nice, especially against things like Dive based decks and growth based decks, former since their 'attack' is worth more to them, latter because they get absurd amounts of it. What happens if the same creature gets soul drained multiple times? Just halved again, or is the other half taken leaving it a 0/0? Former would be more balanced, but if done right and if it could be left at a default 1/1 after two uses that would be interesting.
Also, you want 'halve', not 'half' for that use of the word.