Some sectors at San Jose downtown are dangerous, however the crime as not escalated to the level that you describe. But any person (tourist or not) should avoid some areas for his own safety. The new goverment as increased the number of policemen at the downtown.
About the story of the trail of blood, well it is not true. A few months a person was badly injured in an assault at the city, maybe he saw that... but on a daily basis? no way.. 
Sorry but I am hearing repeated
stories of San Jose being dangerous and they are consistent and concentrated within a few blocks of Avenida Central. The trail of blood story is true, the witness asked at reception about it and another guest was stabbed on his way back from the Morazon casino to that hotel. Another story I failed to mention was another victim who was assaulted and broke his hand, in his account he was not alone either, furthermore it occured with 100m of the stabbing.
Others have reported running from these gangs, getting held up at gunpoint or watching this from the safety of a hotel. The news is out now and many more are taking taxis. But other tourists have cancelled their trips, or switched to hotels away from San Jose, I may just change my destination altogether.
Tourists are obviously an easy target, the gangs know that very few of them will report what happened to the police, especially if they like I am only there for a day before moving on in my itinery.
Unless the news gets out, then nothing will happen until someone goes too far and there's a death.
Some of the hotels in the area are getting together to lobby on this. They have talked to the Fuerza Publica as well as the city police. They have made large donations over the years for equipment etc. None of that has done any good as the donations go to wherever there is a need and not to increase the security of that specific area. Nor can they fund the salary of a cop to work in the immediate area, the donations cannot be made for a specific purpose.
And a final question, how am I, a visitor to Costa Rica to know what areas are safe?
I read http://www.amcostarcia.com many times a week, even that does not help me to know that certain tourist areas are not safe. I've found it hard to find crime warnings about downtown altough I'd know enough to stay away from certain barrios on the way to the airport.