Craig Scully-Hicks, 36, told a family court he had been left “lonely and frightened” since Elsie's death at the couple's home in Llandaff, Cardiff, when she was 18 months old.
Elsie died in hospital four days after being violently shaken and thrown on the floor by Matthew Scully-Hicks, 31, who could not cope with caring for her when his husband was away.
A jury convicted the part-time fitness instructor of murder following a trial at Cardiff Crown Court and he was jailed for life, with a minimum prison sentence of 18 years.
During family court proceedings relating to Elsie's elder sibling - known only as C - Craig Scully-Hicks said he was “numb and devastated”.
In a fact-finding judgement, Mr Justice Moor said Craig Scully-Hicks, an account manager, did not want to be alone in the house where the murder took place.
“He is lonely and frightened. It was his decision to part. He did so as the medical evidence was telling him that Matt had hurt his daughter,” Mr Justice Moor said.
The judge said Craig Scully-Hicks had confronted his husband after receiving a medical report about Elsie's injuries.
“Matt continued to deny that he had done anything but the reports were categoric,” Mr Justice Moor said.
“He didn't know what to think. Over the following days, there were endless heated arguments.
“He accepts that Matt hurt Elsie and she is dead because of it. He said it is a permanent separation.
“He had taken his vows in 2012 very seriously but has removed his wedding ring.”
During the family court proceedings, Craig Scully-Hicks told the judge he will continue to “beat himself up” about being more concerned about Elsie's health rather than how her injuries were caused.
There is “nothing” that makes him think he should have picked up on his husband's behaviour at the time and no concerns were raised with him by professionals, the judgement states.
“The children idolised Matt and he idolised Matt. He said he didn't know this 'monster',” Mr Justice Moor said.
During the trial and family court proceedings, it emerged that Craig and Matthew Scully-Hicks had met in Swindon, Wiltshire, in 2006 and started a relationship two years later.
They relocated to South Wales, where Craig Scully-Hicks had a large family, and married in Portugal in August 2012 - having already discussed that they would like to have children through adoption.
Elsie's elder sibling, C, was placed with the couple as a baby and Matthew Scully-Hicks gave up full-time work to become primary carer - with Craig Scully-Hicks playing a “significant” role too.
Craig Scully-Hicks' job required him to work away from home for two to three nights per week and he was never present when Elsie suffered the injuries.
In November 2014, Elsie was born Shayla O'Brien. Her father was never identified and she was removed from her birth mother a few days after birth.
She was placed with Matthew and Craig Scully-Hicks by the Vale of Glamorgan Council - the same local authority that had managed C's adoption - in September 2015.
Mr Justice Moor said the fact the case was a “gay adoption is quite irrelevant”.
In December 2016, Craig Scully-Hicks informed the family court that he accepted his husband had killed Elsie, as well as causing her a twice-broken leg in November 2015.
“Matt had lied to him ever since. He frankly accepted that he wanted not to believe it and he was numb and devastated,” Mr Justice Moor said.
“Craig denies any failure to protect and says that the man (Matthew) that injured Elsie bears no resemblance to the man he married.
“He had no reason to suspect Matthew. They have separated and there will be no reunification or reconciliation.”
Craig Scully-Hicks, who described his husband as “Safety Boy”, told the family court that the toddler's injuries were “awful as Elsie had been entrusted to their care”.
“He said he did wonder why Elsie was having so many accidents when C was not having them but the professionals had accepted the explanations,” Mr Justice Moor added.
Craig Scully-Hicks had heard his “quiet and mild-mannered” husband raise his voice to tell his children off, but had never witnessed him swearing at them.
In January 2016, he noticed a “couple of marks” to Elsie's ears, the judgement states.
However, Mr Justice Moor said there was no behaviour at home that would have alerted Craig Scully-Hicks that “something was not right”.
“One of the conundrums of this case is that, on the surface, Matthew and Elsie were bonding. She showed no fear of him. She was happy to go to him,” the judge added.
He said there were no “flaws or inconsistencies” in Craig Scully-Hicks' evidence.
“I have therefore come to the clear conclusion that I should exonerate Craig of any failure to protect Elsie,” the judge added.
“There is nothing he could or should have done but I fear he may find that difficult to accept.”
Craig Scully-Hicks moved in with a friend after separating on 17 November - what would have been Elsie's second birthday.