Slow to anger...great in power

I have been reading through the Bible for awhile now, and am currently reading the minor prophets.  Today I read the book of Nahum.  First, I found it interesting that the book of Nahum is about God's judgment against Ninevah.  Ninevah is the city that God sent Jonah to in order to preach repentance.   The Ninevites did repent, and God spared them from His wrath.  However, later, the Ninevites again turned from God and increased in wickedness.  And now God is ready to judge them for their wickedness.  Nahum 1:3 struck me in what I would consider an inconsistency in God's character: "The Lord is slow to anger and great in power; the Lord will not leave the guilty unpunished."  How can God be both slow to anger AND great in power, punishing the guilty?  I can easily jump on board the "slow to anger" part because I want people to not get angered easily with me.  But "great in power" appears to be such a negative aspect, making me think of destruction and hard-handedness.  But the more I thought about it, the less I saw it as a contradiction.  First, I know that God does not contradict Himself, so I knew I had to evaluate further.  But then I started thinking about how much power it takes to be slow to anger.  God's power is demonstrated to me every time He shows me grace instead of the wrath I deserve for the sins I commit, for my disobedience, for when I choose the things of this world over the things of eternity.  God constantly shows His power through His restraint of justice in my life.  But this verse is also talking about how God shows His power through His justice.  God had given the Ninevites opportunity to repent and follow Him.  Even though He is slow to anger, it does not say that He does not get angry.  He withheld his justice from them once when they repented, but this time, His justice would be complete and the Ninevites would be completely destroyed.  One of the last verses of the book says, "Nothing can heal your wound; your injury is fatal."  Does God contradict Himself?  Nope. As hard as it is to understand, He is both merciful and just and I can trust that He knows when to be slow to anger and when to show His power through His justice.  I am so glad I serve a God I can trust and who loves me enough to demonstrate His power through Jesus' sacrifice on the cross to pay the penalty for my sins so that I will not have to feel the full power of His wrath against my sin!  Praise Him!