In this passage, the Jewish scribes had an accusation against Jesus.   

When compared to the reaction of the people who were amazed at his authority to cast out unclean spirits.

The unclean spirits did not question Jesus’ authority, but here we have mere men, the ‘spiritual’ leaders of the people, who are not only questioning but have a serious accusation of Jesus.   Accusing him of casting out demons by the power of Satan.

Normally Jesus answered questions with another question to make people think, but here Jesus answers directly to strongly warn against a line, when once crossed leads to dangerous territory.

Beelzebub was a Philistine god, the “lord of the flies or the lord of filth”, and used in the New Testament of Satan, and Jesus parable responds this accusation by using the phrase “a strong man”…

Division causes destruction.  A house divided against itself cannot stand.

Mark 3:22-30