Nicola Sturgeon has cited Scotland ending automatic prison releases for serious offenders as she called for lessons to be learned from the London Bridge attack during the ITV election debate.
Although criticising Boris Johnson's "crass" attempts to politicise the "horrific" incident on Friday, Ms Sturgeon suggested that the killer should not have been free to commit the attack.
The Scottish First Minister and SNP leader argued that the lessons learned must "do a service to those affected by this atrocity".
Responding to a question about the killings by Usman Khan, Ms Sturgeon said: "He shouldn't have been out – I think we can all agree on that – and there are lessons that will be required to be learned.
"But I don't think we do any service to victims or to the wider public to rush to those conclusions or to have knee-jerk reactions and I frankly don't think Boris Johnson has done any service to victims with the crass way he has sought to politicise this issue during an election campaign.
"The most serious offenders, like this one, should be in jail for lengthy periods.
"In Scotland, we have ended automatic early release for prisoners sentenced to more than four years in prison.
"I think that is right, it should be up to the parole board to determine the safety of releasing somebody."
Ms Sturgeon also called out Boris Johnson's lie during his Andrew Marr interview that Parliament had blocked his Queen's Speech and plans for tougher sentences, when the House of Commons voted in favour.
She added: "There is an obligation here on a Prime Minister to try and bring people together in the aftermath of an attack like this, not to seek to politicise it for party political gain."
The debate, which also featured the leaders of the Liberal Democrats, Brexit Party, Greens and Plaid Cymru, as well as representatives for Labour and Conservatives, moved on to cover election issues including social care, the NHS, nuclear weapons and climate change.
On the subject of Brexit, Tory representative Rishi Sunak accused Ms Sturgeon of being wrong to claim the Prime Minister would not negotiate a new withdrawal agreement and warned that a Labour government supported by the SNP would lead to further political division from another Scottish independence referendum.
Ms Sturgeon responded by repeatedly challenging Mr Sunak about whether he could guarantee that there would not be a no-deal Brexit at the end of the next year if a trade deal had not been negotiated.
She added: "The biggest con in this election is the slogan from the Tories that if they win this election Brexit gets done.
"We only move into the next – and probably more chaotic – part of the process, and of course a no-deal Brexit would be back on the horizon next year."
Mr Sunak replied: "We already have a deal to leave the European Union and of course we will finalise the details next year.
"The point is we are talking about the future and we can only get to that future and move on if we actually first respect the result of the referendum and leave."
When the issue of nuclear weapons was raised, Ms Sturgeon drew applause from the studio audience with her opposition to Trident and confirmation she would not use them.
The SNP has repeatedly called for the weapons system – based on the Clyde – to be scrapped, with Ms Sturgeon describing them as "immoral" and "a massive waste of money".
Liberal Democrat leader and candidate in East Dunbartonshire Jo Swinson argued that the country should retain its nuclear deterrent, but move towards unilateral disarmament.
Labour's Richard Burgon, standing in for Jeremy Corbyn, was asked about whether his party would agree to another Scottish independence referendum, and said: "There is going to be no back-room deals, front-room deals or any kind of deals with the SNP or anyone else.
"We are going for – and I believe we can and will get – a majority Labour government and so we do not have the position of having a second independence referendum in Scotland in the early years of a Labour government."
Ms Sturgeon, who wants another vote by the end of 2020, said: "I'm not asking Richard Burgon or Jeremy Corbyn to support independence, but I would be asking them – if the SNP hold the balance of power – to respect Scotland's right to choose."
"I don't think they would turn their back on the chance to govern just to block Scotland's right to choose," she added.