Fr. Flader on the morality of tyrannicide

http://www.catholicweekly.com.au/article.php?classID=3&subclassID=59&articleID=5789&class=Features&subclass=Question%20Time

In his weekly Question Time column in the Sydney Catholic Weekly yesterday, The Rev. Fr. John Flader dealt with the conditions under which tyrannicide is ethical. Although the first 90% or so of the article seemed to deal with the topic quite well, his concluding remarks contained something rather strange:

Significantly, [the Dominican moral theologian Fr. Germain] Grisez, in giving the criterion to be followed when one is unsure of the morality of a particular course of action, gives the example of Germans “uncertain about the moral permissibility of killing a tyrant, yet convinced that if such killing is permissible, they had an obligation to participate in a plot to assassinate Hitler” (Ibid, p. 287).

Grisez concludes that if such people, although in doubt, thought it more likely to be true that killing Hitler was permissible, they could proceed. All in all, it seems that Hitler’s regime was one where a good case could be made for armed resistance to authority.
[my emphasis]

But isn’t this the error of probablism? Surely one needs to have ascertained beyond reasonable doubt, not merely on the balance of probability, that tyrannicade is the right thing to do before one embarks on that course of action?

Reginaldvs Cantvar
Feast of Sts. Peter and Paul, Apostles, A.D. 2009