So... Imhotep's plan was to resurrect and then defeat the Scorpion King to take over the Army of Anubis. But we saw that once the King was killed by Rick, his whole army turned to dust and vanished. What was therefore the point of resurrection/killing the Scorpion King, if the army was pretty much useless without him?
In the beginning there was nothing, and it exploded.
Remember the conversation in the car with Ardeth Bay?
He tells Rick something to the effect of " I did not mean to alarm the boy. But he has seven days until the Scorpion king rises. Whoever kills the king can send his army back to the underworld...or.... use it to rule the world.
Rick replies: "So that's why they dug up ol Ihmotep..because he's the only guy tough enough to take out the Scorpion King"
Ardeth: "That is their plan!"
So Rick and Ihmotep were battling each other for a lot of reasons but one of them was deciding the fate of the SCcrpion king's army based upon who killed him. Right after Rick runs him through: "Go to hell.. and take your friends with you!".... and they disappeared.
"Obamacare: It only works if you don't!"