a leader is just "one man" and making decisions for an entire country shouldn't rest on his/her shoulders alone,
I don’t know if you mean it literally but even a president is never alone on making decisions unless he is a dictator. The government compromises of a lot of members with different purposes. As citizens, we ought to vote for those who we think can make a difference alongside the president. While the president chooses who he wants on his cabinet.
In some countries most citizens are more corrupt than the leader so I won't put the blame on the leader alone if the economy is bad and if the economy is good I would say it's a collaborative effort from both the leader and the citizens.
How is that possible? Corrupt citizens would elect a corrupt leader so they can continue being corrupt otherwise corruption would be annihilated if the leader means well. A good leader can and should correct his citizens, in my opinion.