Moonwatcher, this is a society that does feel as if it is falling into decay and rottage. Especially when visiting the Harkonnens at home ~ they have that decadent-Roman-nobles-who-party-and-grab-wealth-and-power-while-the-Republic-falls sort of feel to them. The traditional human vices and none of the virtues.
Corruption and perversion in very pure forms.
I can see how Leto thinks of mortality when confronted with the old woman's greed. Greed for power and wealth is rotting the Empire as a whole, and has made the trap on Arrakis possible. Leto himself does not seem to crave power or wealth as such - but he is desperate for enough wealth and power to create security for his family and retainers and that is why he walks into the trap. I do not know if you could validly call that greed, but you could certainly call the whole Spice industry as having been run by the Harkonnens a great example of amassing wealth and power for the sake of amassing wealth and power.
Perhaps if Leto could have had more time he could have won more adherants to a less corrupt (and greedy) system than that of the Harkonnen...
But even so, in the end he might have fallen, as he would have been one man isolated and facing off against the whole venality of the Empire.
Leto's father, Duke Paulus, struck me as being rather stupid and stubborn in the prequel novels.
He liked to take risks, and claimed it was his duty to go into the bull ring to entertain his people and show them that their Duke was brave. 
How is that for a weird justification for doing what you want to do anyway?
[spoilers for the Dune Houses books]
Duke Paulus was killed by that bull whose head hangs on the wall in his last visit to the bull ring. The bull had been drugged to be extra aggressive and crazy by the Duchess, who originated in an enemy House allied to the Harkonnen and which specialized in high tech. Since Paulus was making nice to a rival techno House to hers, she finally had enough, and so assassinated him in this bizzarre way. So much for political marriages, eh? And that is why Jessica hates eating under the head of that bull. Here is the man who taught Leto that he could only marry for politics, and he was killed by his political wife...
So there is all sorts of weird and interesting symbolism interwoven with the oil painting of the late Duke and with that bull head on the wall. They could be seen as a symbol of stubborness and stupidity in an aging man wanting to once again prove his manhood to the masses against the advice and wishes of his advisors and son. They could be seen as a symbol of the perfidy of political wives and/or women in general. They could be seen as a symbol of "go ahead and do your worst to us, but we will still manage to get ours back", because after all, the poor bull, which was only an innocent (though dangerous) animal when all is said and done - was killed in turn after it killed the Duke. And here is the trophy - complete with the blood of a Duke - to prove it.
[/spoilers]