Still can't downvote. The maximum number of downvotes you can give is 4 times your karma, which is zero with zero karma. So you have to keep gaining karma in order to keep downvoting, and a karmassassination would take a while to save up for.
I should point out that there is no comparable limit on upvoting, AFAIK, so it may be possible to make 10 accounts, post 1 comment on each, get +9 on each (by having the other 9 accounts upvote the 10th account's 1 comment), and now you have 10 9 4 = 360 total downvotes available (10 accounts with +9 karma and 4 downvotes for each karma point).
If it's worth saying, but not worth its own post, even in Discussion, it goes here.