Why was Megatron able to kill Optimus Prime in Transformers 2 -- I thought only a could another Prime?

In the Transformers film universe, it is not true that only another Prime can kill a Prime. While Prime-on-Prime battles have occurred in the films, there are other ways for a Prime to be defeated. In Transformers 2, Megatron is able to kill Optimus Prime using the AllSpark, a powerful artifact that is capable of creating or destroying Transformers.