Vaneloppe, there were still many divorces in 1920s but in higher social circles. The point here is, she was on her own for many years when he disappeared so must have got used being on her own to some extent - I'm sure the shame of a husband just evaporating was not much different from a divorce.
And mainly we are talking about post-war period - she was mature and life was different for all then, many widows on their own after war etc. I don't think it was mainly social pressure in this case (who would really judge her if she led such a limited existance) - I think it's more that he had a hold on her mentally, maybe she saw him as superior because he was clever, and he deceived her successfully for a while. BUT she must have been on the edge of rebellion hence he had to kill her.