I certainly don't see the double standard. I think you misinterpreted the whole Trayvon story as some kind of national outcry against white-on-black violence. It wasn't. It was a nuanced story about a man who chased after a "su ious looking" kid and ended up killing him, then was let off the hook and later found innocent of the charges because Florida state law allows you to shoot someone if you feel threatened. It divided the country, and black America was understandably more sympathetic to the Trayvon side because it was certainly fair to ask what exactly was su ious about Trayvon Martin that night.
There may have been racism in the Chris Lane murder and the Memphis murder, but everyone pretty much unanimously agrees that these people are murderers and should be punished accordingly. Justice has been served. There's no controversy. Unless you think there is some sort of black-on-white crime epidemic that is being underreported by the national media, leading to a national ignorance of the threat posed by black people.