So it would be inhumane to put McVeigh to death, but it is perfectly humane to keep him under lock and key in solitary confinement with no contact with the outside world excepting legal counsel? They did something similar to John Gotti when they incarcerated him 23 hours out of the day, with a single hour for out-of-cell recreation. He didn't last very long once they secured that sentence.
Also, I'm not exactly sure what you mean when some people deserve to die, but our government should not be taking someone's life. If there is justification for killing someone (other than in the course of Defense), who has the right to take that life? You might say that Osama Bin Laden may be an "out-of-state" terrorist, but he is not a 1) head of state 2) member of a recognized foreign military 3) a diplomat...all that makes him is a criminal who called in a mass murder from outside the borders of our country. This makes him NO DIFFERENT than Timothy McVeigh or even Ángel Maturino Reséndiz, aka The Railway Killer, an illegal alien who brutally murdered possibly more than a dozen people. Currently he is slated to be executed on June 27th though he does have an appeal filed.
Resendiz is a Mexican national who committed crimes in the U.S. Do we incarcerate him? Does the state of Texas have the right to execute him? Does he deserve to die (to put this in perspective, he heartlessly bludgeoned 12 people -- possibly more -- to death with rocks, sledgehammers, etc. and raped a few of female victims)? Big questions, but if there is a "right" to vengeance, I would rather it not be in the hands of the ordinary person or the the vigilante mob.