Depends on who he is. A war-adicted veteran of a war will gladly cause Chaos and destruction just so he can ride the battlefield and savour the blood of his enemies one more time. A greedy advisor would see the riches the human ships transport: pearls, gems, gold, silver. War brings loot. That would be his true nature. If he feels truly ambitious he could make an elaborate plot. Where the princess kidnaps the prince in secret, the humans retaliate. The mermaid king will be forced to act and a war breaks out. A few months into the war most will have enough of it already. Then the advisor could reveal that the princess kidnapped the human prince, thus giving her father a reason for his war. This way the king looks war hungry and that weakens his status with the people. The advisor, who gained status by revealing the 'plot' would try to take over and thus rule as the new king.

His demeanor or how he presents it would probably be more 'for the good of the kingdom' or 'think of the children'. Great evils often hide behind good concepts like honor and justice.