The revelation wouldn't be very upsetting if Monsegur was acting of his own volition.
But, according to the documents revealing some of the details of the attacks, and the attackers themselves, the attacks were coordinated by Monsegur after he became an informant for the FBI, and the agents that supervised him were aware of them.
In fact, these documents give rise to the speculation that the FBI was really the instigator of the attacks, and that the information stolen by the hackers from those websites and computers was later passed to US intelligence agencies.
Jeremy Hammond (aka “Anarchos”), who was sentenced to spend 10 years in jail for his involvement in the Stratfor hack, confirmed that it was Monsegur who gave him the list of possible targets among which were the websites belonging to foreign governments.
Knowledge of the Plesk vulnerability that they used to do it was apparently provided by a third hacker and, according to these documents, it's obvious that the FBI then knew about it a month before the public was made aware of it.