I realized the controls, so it is not that. The game just felt really chaotic for me. The slimes moved relatively fast compared to the size of the game area and every time I clicked the button, it always left alive two slimes that were next to each other, and then there was not enough time to prevent them killing each other. If no one else has not said the game to be difficult, the problem probably is more in my reflexes than in your game. However, if others have said something similar, you could maybe consider the slimes being slower at first or preventing slimes too close to each other being the survivors, when the button is pressed.