The debate between Mr Sarwar and Deputy First Minister Nicola Sturgeon was a scrappy affair that shed little new light on most issues as a session designed to allow the two to probe each other degenerated into an angry spat.
Among the increasingly ill-tempered exchanges was the pledge by Mr Sarwar that Labour would scrap the bedroom tax - a move to which the party has been reluctant to commit.
The SNP has challenged Ed Miliband to give an immediate, unequivocal yes or no to the question of whether Labour will abolish the bedroom tax, following what it calls a week of confusing and contradictory statements from senior Labour figures.
Labour insisted there was a distinction between a theoretical answer about what the party might do if it took power tomorrow, and what it was prepared to commit to doing almost two years from now.
But the SNP say that on Wednesday Ed Miliband repeatedly refused to say at Prime Minister's Questions whether Labour would scrap the policy in 2015, despite several invitations to do so.
Labour's Shadow Work and Pensions Secretary Liam Byrne told Good Morning Scotland that, before Labour could make a manifesto commitment they had to prove that the policy costs more than it saves.
During the STV debate, Mr Sarwar said: "If we were in government tomorrow, we would abolish the bedroom tax."
In May Ms Sturgeon and Michael Moore clashed in the channel's first major televised debate of the referendum campaign.
Meanwhile, in a Newsnight Scotland interview, Labour's Scottish Leader Johann Lamont was pressed several times on the issue but refused to be drawn.
The SNP claimed that the fact that Scottish Labour's leader and deputy leader appeared simultaneously on different TV stations and made contradictory statements on the bedroom tax policy called into question who exactly was in charge of the party's welfare policy.
The SNP were unhappy at the way the debate had broken down and even Labour sources conceded that the event had been neither enlightening nor engaging for the watching public but put the blame equally on Ms Sturgeon for the aggressive and unsavoury exchanges.
Gordon Macmillan, Head of News at STV, said: "The second Scotland Tonight Referendum Special delivered another lively and engaging debate for viewers and the audience interacted extensively online via social media with the programme trending on Twitter at a national level.
"The format of these programmes is proving to be a great success with our viewers as the second debate delivered a peak audience of 190,000 in line with the viewing figures of the first debate in May."
On the bedroom tax issue, the Scottish Government has already announced that, should the SNP be elected to office in an independent Scotland, it would abolish the bedroom tax in the first year of independence.
In the meantime, SNP-run councils have committed to a no-eviction policy for those affected who are taking all reasonable steps to avoid falling into arrears.
Labour, however, has instead suggested a package of measures to mitigate the impact of the tax and accused the Scottish Government of failing to back this.