You mean why does it turn 359 degrees instead of 1 ?
Rotation is not a best property to use in 3d space. AnimationPlayer doesn't realize if he should interpolate left or right. It will definitely understand Vector3, so use Transform.basis to rotate. You will have to learn about 3d trigonometry, which is very very panfull :)
There is easier option. Create your animations in Blender and export them with mesh