I have hard time believing this, either that he died or his motive. Tax evasion has one of the most lenient punishments of white collar crimes.
That said, he was 75, so I guess even a few years could potentially be a life sentence.
Usually, without aggravating circumstances, federal sentences would run concurrently, though thr multiplicity might push them up within the range permitted. So, lots of nonviolent up to 5 years per count crimes with no particular extreme factors likely adds up to...about 5 years.
Which at his age might still be a sizable fraction of his life, and he wouldn't be the first person to act based on an unrealistic estimate based on what is theoretically possible rather than likely.