I feel that Psalms 66:5 is showing how great and mighty Jah is. And how terrible the judgement is against the wicked, who fight against Jah and his children.
 He ruleth by his power for ever; his eyes behold the nations: let not the rebellious exalt themselves. Selah.
It is a warning to the wicked to not go against Jah because it is impossible to fight against the Creator.
Psalms 66 also speaks about how Jah purifies I and I and saves us from the wicked. But the reason that there is a need for Jah to purify us is because of the wickedness within us. Silver only needs to be tried in the Fire if it is not pure, if it is pure and clean there is nothing to burn away.
I posted a reasoning before that is somewhat related to this reasoning.
the problems of people are caused by people. They always want to blame God, the devil, or anything other then themself.
People are murdered because other people murder them. People are dying from cancer because people poison the earth, water and air. People have no salvation in Jah, because they have rejected Jah.
1 There were present at that season some that told him of the Galilaeans, whose blood Pilate had mingled with their sacrifices.
2 And Jesus answering said unto them, Suppose ye that these Galilaeans were sinners above all the Galilaeans, because they suffered such things'
3 I tell you, Nay: but, except ye repent, ye shall all likewise perish.
4 Or those eighteen, upon whom the tower in Siloam fell, and slew them, think ye that they were sinners above all men that dwelt in Jerusalem'
5 I tell you, Nay: but, except ye repent, ye shall all likewise perish.
Repent isn't about asking forgiveness, it means to turn away from wickedness. So Christ was showing I and I that even if we are not the worse sinners, as long as we are sinners and separate from Jah, these things can happen to us. And the reason for this is that if we reject Jah, then Jah will let us be our own defence, because we have rejected Jah defence.
Haile Selassie I explains this further,
However wise or however mighty a person may be, he is like a ship without a rudder if he is without God. A rudderless ship is at the mercy of the waves and the wind, drifts wherever they take it and if there arises a whirlwind it is smashed against the rocks and becomes as if it has never existed. It is our firmm belief that a soul without Christ is bound to meet with no better fate.
So if I and I repent and live for Jah, and accept his guidance and direction, then Jah will be our rudder. So just like a rudder guides a ship through the sea, when I and I accept Jah, Jah will guide I and I through this world. So when the waves come, which is the bad things that happen in the world, I and I will not be moved. Jah will keep I and I steady and will take I and I out of the waves.
Haile Selassie I