Why does Bertha's insanity render Rochester's bigamy less unacceptable?

I have been thinking that many Victorian readers would have had some sympathy with Rochester: married to a mad woman who wants to kill him. It is almost understandable that he would try and make a bigamous marriage with a woman he truly loved. But what if Bertha had not been a homocidal maniac, just rather an unpleasant woman? Or if there was nothing really much wrong with her, just that there was a personality clash between them, or they had fallen out of love and had started to grate on each other's nerves. Not many men were married to psychotic and dangerous women, but lots were trapped in loveless and unhappy marriages. However if Rochester's marriage had been of this type, his actions would have been totally unacceptable, and surely Jane, being so pious, would not have gone back to him even after Bertha's death.

I remember watching a programme called 'Who Do You Think You Are?' in which famous people track down their ancestors. In this episode, Kim Catrell from Sex in the City discovered that her grandfather, I think, had abandoned his first wife and family and had bigamously married another woman. You could sense the waves of loathing and disapproval. However, divorces were not easy to obtain so bigamous marriages were not uncommon. Having said that, he did sound like a scumbag because he had responsibilities to his children, which he did not live up to. At least Rochester does provide for his dependants, but then he can afford to.

I think we also have a bigamist in our tree. My husband's grandfather, we think, was married in Poland and then allegedly suddenly left Poland (that is a fact). Hey presto, someone who produced children suspiciously similar (I do mean spooky, same gestures, same tastes) with a name that does not exist crops up in the middle of WWI in England. Hung up a story about a nasty aunt in Marseille and being born somewhere out of the way in Georgia. hmmmm He seemingly married happily a woman he took off another man (:D) and had another 8 children (?).

I do think on a human level, Rochester's case is sad really. As a seeming bachelor, he is supposed to be missing homely comforts supplied by a wife. A Victorian home without a wife is not a home. As Jane Austen put it in satirical terms: Rochester is in possession of a good fortune, therefore he must be in want of a wife. As a forty-year old probable got mothers and young girls swarming around him and he can't say why he is or should not be interested. That's temptation and anguish for you.
Although I still believe it is his fault for not surveying the goods properly upon acceptance (who marries a woman he hasn't even spoken to in private?), Mason's conduct wasn't really on either. You just don't shut up about things like that, certainly not when it is believed that things like that are hereditary) and knowlingly make someone really unhappy.
The problem with the marriage and why Rochester could not divorce his wife is quite sad: the marriage had allegedly taken place when Bertha was sane, i.e. compus mentis, she knew she was committing herself and wasn't forced, a condition for a valid marriage. The marriage therefore is not null and void on the grounds that one party was not accountable due to mental problems. After some time (often alluded to as promiscuous), Bertha is declared mad by doctors, which, unfortunately, makes her unaccountable for her promiscuity (she doesn't know what she is doing because of mental health problems) on the ground of which he could have divorced her if he had had the money. That would have had to be done by private act of parliament. I don't know how much this cost, but it would have been substancial, although by then he had a good estate in Thornfield. Anyway, he can't divorce her because, in effect, she has not committed adultery, so he is stuck with her until she pops off. That's quite sad in a society where things are quite segregated, where there is no casual sex apart from prostitutes and women of easy virtue like Céline (with the normal risks attached). As he says, mistresses are only there until they find someone else and are normally not the most intelligent of women. The most intelligent find a husband, after all.