It depends on how it's spend, and the culture of the good kingdom.
There was a King in history who went to another country and spend a crapton of gold. This caused the value of gold in that country to go down and with the value of each coin tied to the value of gold it meant everything needed more coins to buy. 2 copper or whatever for a full bread before the gold came in? Now it costs 4 silver. The people who had sold stuff to the King and had gold didnt have too much problems with this, but anyone else would suddenly need to pay multitudes more without earning more or having more in reserve.
Suppose the bad kingdom tries to exploit this: it pays
specific groups of people money for wares and stuff, and ignores other groups. Just living becomes more expensive for everyone which will cause strife in the population. If the divide grows quickly enough and large enough people could revolt, destabilizing the country and making it an easier prey.
While you are at it, buy specific goods. Buying weapons might not be the smartest move as the sellers could just have an easier time getting more weapons. Buying all the ore and resources to build weapons could deplete the good kingdom's resources while destabilizing the economy, and if you offer a higher price for the resources than good kingdom then the people who own the resources would gladly hold out and wait for a bad kingdom guy to come for their goods.
Other resources to buy would be food, it might be the best choice. Buying food directly means more stockpiles for you, higher food prices for good kingdom coupled with massive inflation and less food for good kingdom to prosecute a war. Even better: if good kingdom tries to prosecute a war food prices will rise anyway on top of scarcity and inflation, it would probably kill the good kingdom the fastest.
Lastly a risky option: pay the military of good kingdom. If you are rich the rewards of combat pay suddenly is a lot less motivation to risk your life. A portion of Soldiers of good kingdom might just leave when push comes to shove and try to be alive and spending somewhere.