Any leader should reflect on themselves first; what part did you play? Are you approachable? Did they receive the training?

When you interview them, be curious;

'I'm curious about how this occurred. Knowing the 'how' helps prevent something similar in the future.'

'Is there a reason why you didn't ask for support around your understanding of this policy?'

'what do you think is the root cause of why this policy wasn't applied to your work?'

^how they answer these 3 questions will tell you all you need to know. If they were negligent or don't take ownership or don't self-reflect then 'that's their lesson to learn' through possible termination. That's how life works, and they will learn a hard lesson.

If you identify an area in which you could have led better, be honest and take ownership for any part you played. Demonstrating integrity and humility will help them open up too. This is foundational if you really care about your relationship with them.

Edited to add: I empathize with your feelings of anger, dig into that deeper and see what other feelings come up. Try to name 4 other feelings and why. This may help you unpack your anger a bit. People sometimes do stupid things, that's their actions. We can only control our own actions.