There isn't a need to hear Anna's side. This is not real life, so this is the way CDPR used narration to relay the story. Hearing Anna's version would have been impossible for the direction of the story line.
It is implied that he was a kinder, gentler man before the war and that he and his daughter were extremely close and he and Anna were very much in love. The Baron went off to war and lived through atrocities, turned to self medicating (drinking) to numb his pain, he returned with PTDS, only to find that the loving wife he had left is now with another man. In a fit of rage, and possibly due to his mental state of mind, killed her lover. Anna flew into a rage herself and tried to kill him. He beat her and thus started the cycle of abuse for them both.
Anna would belittle him constantly and hit him regularly and he would return that abuse. Anna is the worst offender because she used their child for a tool to abuse. She knew the Baron loved his daughter and that his daughter loved him. It is stated that when the Baron went into a rage (no doubt due to PDTS and constant abuse from his wife, topped off with alcohol) Tamara was the only person that could calm him. Anna hid the abuse she handed out to the Baron and made certain that her daughter was witness to her abuse. Parental alienation is a common thing in domestic violence because it emotionally cuts deep. Tamara was the only thing the Baron loved, so Anna removed it.
Since no where is it implied that the Baron forced himself on Anna, she most likely got pregnant the old fashion way, leading to the narrative belief that there were times of peace between them. When she got pregnant, she didn't want his baby, not because she was afraid for the baby's life, but most likely because of her hatred for the Baron and this was just another way to strike at him. She went to the crones for an abortion, yet it is implied that she led the Baron and Tamara to believe he was the cause of her "miscarriage". Anna's docile and meek behavior when we meet her isn't due to abuse, but because she "danced with the devil" by making a pact with the witches. It's more likely than not that she fled into the night not because of the Baron, but because she knew the Crones would be coming for her.
Does all of this suggest the Baron is an innocent spouse? No. He was a vile man who could have thrown Anna out, but he didn't. They both got caught up in the abuse and punishment--her because he killed her lover and him because she had a lover when she was suppose to be keeping the home fires burning. She hated that their daughter loved him, he hated Anna for turning Tamara away from him. It was a constant cycle of abuse and the only victim is Tamara.
It's a complex mission. There is no "good victim". Both are responsible for their own misery. Helping the Baron isn't really a priority for Geralt, finding Ciri is and the Baron holds that key for now, and I imagine Geralt feels a bit obligated for helping the Baron, due that he saved Ciri and was good to her. The two outcomes are also very poignant, but both have very harrowing results. If you save Anna, the place goes to hell, but the Baron is a man redeemed. His family becomes his number one priority and in doing so, his village falls to a ruthless man. If he kills himself, his selfishness leads to the same result, though if he lives, there is a chance he could return with a healthy Anna and regain his village and as a redeemed man, lead it into a better place.