K.S.
Your husband needs to relax. Kids do stupid stuff all the time. They don't have the life experience to think through the consequences. And even when they understand the possible consequences they often don't think in the moment.
For example my 8yo knows what could happen if he leaves his guinea pigs out of their cage unattended (we have 2 cats). He left them both on his bed this weekend and then got distracted by something else and forgot them for hours. It could have been really, really bad. Fortunately the upstairs cat has decided the piggies are his friend. The guinea pigs were scared and one had fallen off the bed and was hiding, but both were unharmed. It gave my son a good scare though and made a bigger impression than me lecturing him. I'm sure your son won't repeat his mistake either!