Ideally they wouldn't be able to bring the network to it's knees like that, but sometimes one user behaving how they aren't supposed to can highlight areas of improvement in the network configuration.

As JackbyDev@programming.dev said, they knowingly did something illegal that isn't work related on company property and caused an effective outage in the process, which on its own can be a fireable offense, but if performed by an employee who was already on thin ice, it's an easy out for their manager to get rid of them of they didn't have enough reason to let them go beforehand