Why Rocco?
Only FFC or Puzo or a very few others could really speak with authority.

The obvious allusion is to the murder of Lee Harvey Oswald. It may not be any deeper than that. ohwell

If it were any deeper than that my guess would be that Michael wasn't convinced that Rocco still wasn't somehow involved with Roth (after all he was in charge of security) and wanted to test/punish Rocco by making him handle the job personally.

It doesn't seem to make sense as most people know that Rocco was at the very least linked to Michael but maybe the viewer was supposed to think that at this point Michael's influence is so strong that he can get the media or authorities to ignore such things.

But this of course is ALL wild speculation. rolleyes
I really don't think it goes much further than the homage to the LHO murder.

"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming

Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.