Saleen did not originally have plans of massacring the plane, but when he realized that the water-type element spirit actually intended to become a god, he decided to plunder the plane until nothing was left.
While the water-type element spirit was caged securely by the Lightning Cage, Saleen nonetheless feared that it would spark a divine fire. The cage did not truly isolate the water-type element spirit from the outside world after all, and that was the Lightning Cage's weakness.
If the cage contained something that had no external support of any kind, there was no doubt that the Lightning Cage would be the most difficult thing to break into in the world.
To put it precisely, the cage was virtually impossible to break out of from within.
