It is a simple, if tired point. Do you want a Prime Minister who confronts and defeats terrorists, or one who sympathizes with them?
Inside that question are many more -if it was wrong for Jeremy Corbyn to invite Gerry Adams to the House of Commons, was it wrong for the government to be talking to representatives of the IRA at the same time? Legally, yes, Corby was at fault, because the IRA/Sinn Fein were guilty of crimes against the UK and its citizens. Morally? Difficult, because it may have suggested to the IRA/Sinn Fein that they would get a better deal from a Labour, than a Tory government (as indeed was the case).
If the USA describes HAMAS as a terrorist organization, why was that organization when it started up given so much support by the state of Israel, an ally of the USA?
If Corbyn was soft on a United Ireland, sympathetic to Hugo Chavez in Venezuela, is it not also true that Margaret Thatcher was a supporter of and gave a personal welcome to General Pinochet in 1999 even though his regime tortured a British citizen- Sheila Cassidy (the secret police had trained German Shepherds how to rape a woman when she was place in a prone position) -and did not that same party regard the ANC in South Africa as a 'terrorist' organization'?
Corbyn's position on the Falkland Islands is indeed weak, I recalll at the time how the left ridiculed it as an imperialist adventure. It was in the early stages seen as crucial to ousting Thatcher from power. But if you step back and think, it would have made sense for the British and Argentine governments to agree a joint sovereignty arrangement which gave Argentina and the UK an equal share in the oil and gas resources of the Falkland/Malvinas basin (identified c1971), gave citizens equal access (because 99.99% of Argentinians are no more interested in invading and occupying the Falklands than the English are in living in the Outer Hebrides), and made all war pointless because both states were in agreement that co-operation makes more sense than conflict.
And if it is the case that Corbyn cannot control his own party in Parliament, bear in mind there are ferocious beasts in the Conservative Party who unlike Theresa May have hated the EU even before the UK joined in 1973 and regard her with suspicion. They need her to win the election, they don't need her to carry on the negotiations. Look closely and you will see that there are as many cracks in the Tory party as there are in Labour.