Trump knows he’s lost the election, and he knows he has no hope of ever being elected to anything — ever. He has absolutely no loyalty to the GOP (or any political party), and is just going for scorched earth at this point (because that’s what tantrum-throwers do).
Rubio is acting as the adult in the room (amazingly), and he’s absolutely right. Both political parties should be concerned about foreign forces attempting to influence US elections, and if the Republicans eagerly seize upon exposure of leaked emails of Democrats provided by Russian hackers and use it to their advantage, it tells the hackers that their strategy works. And it could very well be the Republicans next time. Rubio and other Republicans depend on the reliability and public faith in our elections for their careers, and they aren’t anxious to allow it to be destroyed.
Also worth mentioning — taking a close look at the emails within a campaign is a little like watching sausage being made. One gets to see a lot of stuff that’s normally hidden from view, but goes on in any sausage factory (an examination of any candidate’s staff’s communications will also show a lot of standard campaign strategy conversation that may look a bit foreign and even startling to those who don’t know what goes on in a campaign). However, so far there are no smoking guns here. That won’t stop Trump, of course — even if he has to photoshop the smoke in himself.