Some of them said, "By the power of Beelzebul, the prince of demons, he drives out demons." Others, to test him, asked him for a sign from heaven. But he knew their thoughts and said to them, "Every kingdom divided against itself will be laid waste and house will fall against house. And if Satan is divided against himself, how will his kingdom stand? For you say that it is by Beelzebul that I drive out demons. If I, then, drive out demons by Beelzebul, by whom do your own people drive them out? Therefore they will be your judges. But if it is by the finger of God that (I) drive out demons, then the kingdom of God has come upon you. (Lk 11, 15-20)
The opponents of Jesus were looking for a hidden truth behind the doings of Jesus. Externally Jesus was casting out demons and they accepted it. But they were reading or putting intentions into it. They could not read the intentions behind it.
The intention behind it is clear when Jesus said: “then the kingdom of God has come upon you”. This was the intention of Jesus and he did not have any need to explain it or give sign from heaven to prove it.
The intentions which the opponents were reading or putting into his actions were: "By the power of Beelzebul, the prince of demons, he drives out demons."
Because of this Jesus had to explain saying that a kingdom divided against itself cannot stand etc.
The intentions of the doer is important and not the intentions of the viewer. For the doer knows why he does something and for what reason he does it. A viewer can have his own intention and it could be totally different from that of the doer.
Whatever Jesus did was with the intention of bringing in the kingdom.