It was a bad idea in 2016. It is still a bad idea.
Well, I cannot fault Donald Trump for sticking to his campaign promises. The problem is that one of his promises - federal criminal penalties for assaulting for killing police officers - is a really bad idea. Assault and murder are local crimes, not federal crimes. Therefore, they should be handled locally, not by the federal government. A new law is totally unnecessary, because people who assault or kill police are already prosecuted and punished.
Republicans spent most of the 1990's arguing for a limited federal government and returning power to the states. The Tea Party movement reinvigorated the push for federalism. Sadly, when a Republican is elected President, too many Republicans suddenly think expanded federal power is a good idea. We saw it when George W. Bush was elected in 2000 and we are seeing it now under President Trump.
Republicans have not just flip-flopped on state sovereignty; they have flip-flopped on "hate crime" laws too. Making it a federal crime to assault or kill police officers operates under the exact same logic as "hate crime" laws. How can Republicans maintain credibility in opposing "hate crime" laws while pushing a federal law that operates under the exact same logic?
The push for a new federal law protecting police is a panic disconnected from facts. Police are safer now than they have been in generations. Crime overall is far lower than it was 25 years ago. The fearmongering about violent crime is simply not based on reality. Unfortunately, Democrats will likely roll over and go along with this instead of being a principled opposition for fear of being labeled "soft on crime."
There is some overlap between populism and conservatism, particularly on putting American interests first in foreign policy and a strong national defense, but this is not one of the places where populism and conservatism agree. Republicans and conservatives need to resist the big-government impulses of President Trump and continue to push for a smaller, more limited federal government.