Ouch. I was all set to go with murderers' until I read the post by Miller. Each of those guys excelled at his job, so maybe it should be murderer's. But if it was a team effort, go with the s' in my opinion.
I looked it up on google for a long time, and decided on murderers' based on my own logic and some key links, including the one Jespah gave from the Hall of Fame, another one from the Baseball dictionary, an official seeming Yankees' website, one from the New York Daily News, and others. I think I found about twenty corroborating the murderers' before I stopped clowning around and got back to work.
After all, there was more than one "murderer" in that row in the monoprint this artist did. In his picture, it was Ruth and Gehrig (though I may have the batting order wrong).
Thanks for the nice link, Miller.
Osso, You will frequently see plurals rather than possessives in such a situation. For example, the Writers Union, the Authors Guild. I admit that my impulse upon seeing these is to rush in and add an apostrophe, but I think it can go either way.
One is simply stating the name of a group. The other is applying possession to the group. In the case you're citing, you gotta go with what is historically accurate. If you find the apostrophe in documents of the time and in the Hall of Fame, then apostrophe it is.
Yes, I see the possibility of either way... at least in prevalence of usage, in that google also had links with no apostrophe after the s.
All's well that ends well; we're on onto new concerns.