To me, there's a bit more selfishness (borderline jealousy) in Russell tapping Frank to kill Hoffa. He knows how conflicted Frank is about them offing his friend, and while he says he wants Frank to do it so that he's settled, it signals to the bosses that Frank is their guy and they don't have to worry about him, but following that thread, it shows the bosses that they don't have to worry about Russell. That's why in jail he's like "it was him or us." It wasn't just about saving Frank's life. It was saving his own.

But under all of that (And why I say borderline jealousy), I think Russell in the film (not sure if in real life) wanted to be assured that Frank loved him more than he loved Jimmy.

So like I'm saying, I totally agree that he was saving Frank's life yet again, but I think there's a layer of self preservation there that can't go unlooked.