I believe power corrupts, though not immediately.
You may be the most pure and virtuous soul on the planet when you first come in to power and you use your new-found influence for the betterment of the people. They cheer for you, thank you, adore you. It will eventually go to your head because you're human, not perfect.
At some point, you will start making decisions that you think are in the best interest of the people, but in reality it only makes them miserable. You strip away freedoms they've enjoyed for years for security reasons because you want to protect them...but the people you rule see only a tyrant who has lost touch with his subjects. They voice their concerns and your first thought will be something like "they're just fools who don't understand my grand vision" or "they're troublemakers, looking to incite a rebellion" or something similar.
How do you respond? Do you take a step back, do some introspection, and see that they're right? Very doubtful; you're the king, you know what's best, and damn it, they have no right to question your wisdom! Haven't you always kept them safe? You're doing this all for them! But clearly, they are some problems that need to be dealt with to ensure the peace...they will need to be taken out.
What makes it worse is that you will inevitably be surrounded by assistants, some of whom will become corrupt themselves. They will support whatever mad idea you have, try and make you suspicious of the ones who still speak for the people, push you along your dark path. Why? Because the more power you have, the more power they have. Greedy, amoral yes-men are a dime a dozen and while some can be easy to spot at a glance, others simply start out trustworthy and then devolve into power-hungry dicks.
You may believe you're doing the right thing up until your dying breath, but you have become corrupted.
If you don't like the mantra of "Power corrupts; absolute power corrupts absolutely," then I would instead offer "You either die a hero or live long enough to see yourself become a villain."