I doubt that he meant to do away with trials, and my bet would be that it was just pandering, not ignorance. The death penalty is already on the table for killing Federal LEOs thanks to the "it's a Crime, Bill!" of 1994; about the only thing he could actually do is require Federal prosecutors to seek the death penalty in cases involving the death of a Federale instead of them having prosecutorial discretion.
Personally I think it's a great idea. It'll cut down on plea dealing. Either prosecute your case or don't bring it at all.