Something about Sally Lucas bothered Turlough from the moment he met her. Maybe it was how hostile she was - it seemed disproportionate to the situation, even to someone who’d been traveling with Tegan for months. But the Doctor believed in trusting people, so Turlough ignored his instincts and followed the Doctor's example instead.
And here’s where it had landed him: being threatened with electrocution by a turncoat who he himself had armed. Turlough held his breath as Sally aimed the sparkwire at him, and sighed with relief when it backfired on her.
Trust was nice, but temporal grace was better.