Using the Cingranelli-Richards (CIRI) human rights dataset and the Political Terror Scale, Xavier Marquez produces a quantitative history of political regimes, examining malevolent democracies and benevolent autocracies:
If anything, a slight worsening trend in the extent to which states engage in torture, killing, and so on is detectable here, despite the increase in democracy over the same period.
His conclusion:
At any rate, it seems as if the old idea of checks and balances is at least somewhat vindicated by the evidence of the last three decades: constraints matter, and don’t count on benevolent autocrats.
The failure here is to assume a static model — that democracies remain democracies, and autocracies never become benevolent. Actually the main problem with democracies is their tendency to devolve into the worst kind of autocracies.