He was executed by the government of Bulgaria, by this sentence (via Wikipedia and this site, note links oddly placed in lower-right corner):
By virtue of articles 70 and 83 of the penal code, the court condemns the accused, Eugene Bossilkov, to be sentenced to death by firing squad, and all his goods confiscated...Dr.Eugene Bossilkov, Catholic bishop; completed his religious studies in Italy and was trained by the Vatican for counter-revolutionary activities and espionage. He is one of the directors of a clandestine Catholic organization. He was in touch with diplomats from the imperialist countries and gave them information of a confidential nature. The accused convoked a diocesan council in which it was decided to combat communism through religious conferences, held in Bulgaria, activities called 'a mission.' No appeal of his sentence is possible. The High Court, Sophia, Bulgaria, October 3, 1952
The sentence was carried out 60 years ago today. That's not so long ago. Switch out Communism for some of the errors of our own time which one is not allowed to question, update 'mission' to 'New Evangelization,' and the same sentence would be ready for new martyrs.
I was brought up with the standard liberal doctrine that the 'separation of church and state' was for the protection of civil government from religion. And I wouldn't deny that sometimes this is a good thing. But it's also to protect people's legitimate desire to be faithful to God from the excesses of human power, which so easily decides that it doesn't need to refer itself to its Creator.
Blessed Eugene, pray for us.