In the Costa Rica incident referred to, the former Marine was able to get a choke hold and kill the robber because it was a close quarters combat scenario. The guy got too close to the victims and paid for it with his life. As I recall they had boarded the tour bus and were in the aisle. Bad planning.
However, it is important to note that if that old Marine had failed he could have caused the robber to start shooting - either out of anger or panic - and tourists could have died. Frankly he took a huge risk with other people's lives.
It bothers me that I still see media reports making the St. Thomas murder a cruise-related crime.