First, on using the JW joke as a sales opener. I would not recommend it. A religous joke like that comes across as unprofessional and throws flags up right away that makes people think "oh brother, a snake oil salesman." The benefits of it are minimal and the chance of it backfiring are too great.
Second, I can understand your reaction. When you expect people to laugh and then you have someone lash out at you can be a shocker and its easy to react before thinking it out. From his perspective though, perhaps his grandmother is a JW and he knows a bit about it and considers himself one. Or perhaps he is a hypocritical JW. Either way he was obviously offended by the religous joke. Ridiculing him in front of people for being offended at a derogatory religous joke does seem quite rash. I think an apology and explanation would be a very kind thing to do, and perhaps you will get the same from him in return. If you don't like the judgemental attitude of the JWs then be sure to act exactly the opposite.
I'm not surprised with all the "hurrahs" and "go get 'em" from this peanut gallery. Its actually quite scary. I dread the thought of JWs being outlawed by a facsist government, I'm sure all the voluntary executioners will come from this DB.