Here is what you said.
Let's say that you're a religious Methodist, and some rabbi comes along and says that he now converted a dead relative of yours to Judaism: is that ok with you?
A rabbi would never say this. Because you can't convert the dead by his beliefs. A Methodist would not believe him because you can't convert the dead by his beliefs.
You really are a member of the club of the perpetually offended aren't you?
The Mormons are baptising dead Jews; essentially making them something that they never wanted to be when they were alive: Mormons or Christians or anything else but Jews.
They are taking their memory (for instance, Anne Frank has been baptised posthumously by these people) and changing it among the living. That is offensive to people who hold the dead's memories' dear to them. If you cannot understand that you are hopeless. And on a personal note, I am an atheist who does not believe in the afterlife. So for me, this is all meaningless; but I do understnd how Jews who are serious about Judaism can be bothered by this; they used to force baptism on Jews and put them to the sword if they did not convert.