Mueller and Rosenstien, and if not going to get with the program, Sessions, need to be removed unceremoniously and placed out next to the curb for pick up.
He had a brain tumor, not "has" a brain tumor. Actually, he had a blood clot... which they removed. Since this type of cancer is associated with clots they biopsied the removed clot out of an abundance of caution and discovered the cancer cells.
They caught this in a very early stage, but they'll likely still give him chemo to insure any other cells are wiped out before they can grow into tumors.
I sincerely doubt this fast moving cancer had anything to do with his behavior for the past many years that he's been doing what he's been doing... his anti-Trump stuff last year, or his involvement with Syrian "rebels", or his involvement with Ukraine over the past several years, or his performance in 2008 and the subsequent back-stabbing of his running mate Palin, or any of the many other bad things he's done throughout his career.
Bottom line... he's always been a self-serving, power-crazy SOB. As serious as it is to find even early stages of this cancer, we can't blame who he is on this illness.
And yes, the right thing for him to do would be to resign now. The chemo will I suspect be pretty draining for many months and he won't be able to function as a senator very effectively. Of course he won't resign though, since he sees his job as obstructing (which he can do by not being there to vote) and to verbally attack our president, which he can do from the comfort of his home.
In fact, he now has a new card to play... he can take shots at our president all day long and he's somewhat protected from return fire because we're supposed to have sympathy for him. This is almost as good as the race card, but its a card none-the-less... and he'll use it as best as he can.