Untrustworthy... for doing exactly what he said he'd do before the election? 'Seek to form a government with the party with the most votes/seats'?
You can hardly blame him for talking to Labour as well, what if the talks with the Tories had fallen apart and he'd had to start from scratch with Labour/ An extra 3-4 days of no government.
What exactly do you think is untrustworthy?