I am sure you are aware that not all of the Digimon you encounter in this world are going to be friendly. But some....some have been affected by what is known as a black gear. It empowers evil Digimon, and causes the formerly good to lose their minds and attack without thinking.

But the Digivice-

[He points at Kovu's digivice, which is still on the rock where he dropped it.]

Is able to dispel the gears and destroy them.