Smart.

...the atheist stalinist regime did an efficient job of using up just about every other mode of execution that might have been featured on this t-shirt. I do not intend to assert that atheism causes one to brutally murder millions of people, but only to point out that, if this t-shirt intends to assert that atheism prevents one from committing such heinous acts, it ought not to attempt such a claim by implying an un-stained historical record on the matter.

Louis-Alexandre, duc de La Roche-Guygon et de la Rochfoucald d'Anville: stoned to death in front of his wife and mother during a secular-atheist french revolutionary riot. Presumably there are others too.