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Antonio Cassese has always defended the idea that internationalists’ role is to fill the lacunae of international law by putting humanity and its values above the will of states. As a judge, this conviction expressed itself under the form of judicial activism. This article focuses on his treatment of the question of terrorism as a scholar and as President of the STL. In fact, while A. Cassese had been defending the idea for a long time, in February 2011 the STL issued a controversial decision in which it asserted the existence of a customary definition of terrorism. The study tries to understand the underlying objectives of the decision re-contextualizing them into A. Cassese’s general thought. It shows that the aim of the decision was to push the evolution of international law, although its ability to do so remains uncertain.