NEVER run a series motor without load. It will accelerate towards infinity, worst case the anchor will disintegrate.
As it is now, it's running at far too high rpm, which explains your excessive current draw.
While, in THEORY, the above is true, it neglects windage and armature iron losses, which both increase with speed.
So, unloaded series motors will generally not explode or be damaged without load, but they will run at the highest speed where those losses reach equilibrium with the energy coming into the armature.
Another point: series DC motors are very often timed for a certain rotation direction and will not perform well if reversed.
Series motors will ALWAYS run in the same direction, regardless of power supply polarity!
Unless you have switching to reverse the respective polarity between the field and armature, they will always run in the direction designed by the manufacturer.
Jon