Dr. Marc Shapiro has a fascinating post - which includes a discussion of the Grossman execution and Tropper at the end
SeforimBlog
SeforimBlog
[...] During the discussions about the Grossman execution, I looked at some of the haredi websites (until the comments made me sick). What I found interesting was the incredible level of ignorance of most of the writers, all of whom had been in yeshiva and many of whom had studied there for years. They were able to declare that a murderer can’t be executed unless he was observed by two kosher witnesses and was given warning, which they thought settled matters. Had these people known a bit of responsa literature, there would have understood how things worked in the real world, and especially what was done in the days of the rishonim. Do these people think that if a guy stood up in shul and opened fire with a machine gun, killing 20 people, that a Jewish court couldn’t execute him because he was never given a warning? Let’s continue with R. Liebes:[...]