Coordinates bugs in commands

Stranger Wise, if I execute

/kill @a[x=-518,y=102,z=-1019]

it kills me everywhere

And also this only detects whether I am riding a minecart or not:

/testfor @p[x=-507,y=-1008] {Riding:{id:MinecartRideable}}

And also this:

/testfor @p[x=-507,y=108,z=-1008,dy=80] {Riding:{id:MinecartRideable},SelectedItemSlot:0}

always has the output: "[20:51:23] The entity UUID provided is in an invalid format"