Wouldn't have happened if things were solid. We - along with others - have had this discussion before in your previous thread. Nice cards? I can show you some very, very nice cards given to me less than a month before BD whilst she was in the midst of her affair. But we won't rehash all of that again. The thing is, the seeds of doubt were there long before, and are exemplified in her having those thoughts over the years. Happy/committed people don't have those thoughts.
Not trying to bust your chops, my friend. It is what it is. The important thing is for you to stop trying to figure out the whys and what-fors, and concentrate on yourself...and yourself only. Regardless of when the trigger was pulled, the round was loaded long before that.
There are moments in this life when you are so confident in the rightness of your actions, that not even for a second do you consider the option that you might be wrong.