IT's a tight rope to walk, but there's a system to the madness in regards to presidents: if you punish them to the degree that (parts of) the public demand, you'll almost certainly make it a losing proposition for any of them to ever resign.
African countries have traditionally been very good at demonstrating this, unfortunately. Almost any leader of government south of the Sahel and north of South Africa from independence to about 10 years ago could have been thrown into a dark abyss without second thought and even Amnesty International would have just muttered a "Wait, they deserve a fair tr.. oh! never mind!".
But because that actually was happened more often than not, each one of them clung to power for as long as possible – which in some cases was (and is!) surprisingly long: Mugabe is at it since 1987.
In many cases, these dictators actually started out somewhat promising. Mugabe, for example, came to power against a backdrop or actual injustices, which he actually did something about (although what he did resulted in disaster). But spend enough years with absolute power, surrounded by yes-men, but constantly fearing the yes-I-have-knife-man and it starts to do strange things to your psyche.
It's not the only mechanism – there are enough examples in other countries of politicians overstaying their welcome. And it doesn't quite apply as neatly to anyone but the very top. But it's part of it.
Which is why it was so encouraging to see how the crisis in The Gambia was solved a month ago. You didn't even hear about it? Well, a decade earlier, you wouldn't have heard the end of the bloody war currently ravaging the west African country of... Part of the solution was guaranteed protection for the outgoing guy (+family and his mysteriously large wealth).