Re:One law for them, another for us.
There are clear policy reasons for this. Police put themselves in real danger every day and are effectively the last line of defence between civilised society and anarchy. So it would make sense to make an example out of those who kill police, because you are not only killing somebody, you are making an attack on civilised society as well.
But I agree with what you are saying, because the extent of the discrepancy in this particular instance is exaggerated. It says more about the leniency towards the majority of murderers, which is a bit of a disgrace really.