This is a classic prisoner dilemma that our society finds itself playing again and again. Develop the technology, and you unleash a pandora's box for the whole world. Refuse to work on it, and may be the other guys still does it: so pandora's box is open anyway, but now you've got the short end of the stick.

Thankfully, humanity already tried several different ways to solve such problems. However, I don't remember open letters being one of the effective ones.