If you feel the marriage ending is inevitable I suggest you end it as quickly and cleanly as possible. My reasoning about that relates to an aphorism someone told me a long time ago.
"There could be a sound reason why you need to cut off a puppy's tail. There is never a good reason to cut it off centimeter by centimeter"

Now, if you know what you need to do, just do it. You do not need to be perceived as the good guy when you do it. Its inevitable that you will be judged harshly, especially by spouses family. A divorcing spouse is nearly always going to have in-laws and friends think poorly of what their doing. There isn't a way around it as far as I can tell.