They have to be alive.
That Slate article doesn't say what you think it says.
A pre-emptive pardon can only be issued for someone in danger of being convicted of a crime; it is essentially a grant of immunity. A dead defendant who was merely charged with a crime can never be convicted of the crime, so a dead defendant cannot be pre-emptively pardoned.
http://www.slate.com/articles/news_and_politics/explainer/20...
but I don't think he should be, it really solves nothing in this case.