Love for Enemies
The practical reason why you should not harden your heart towards your enemy I think is because it may just well be that your enemy recognizes their sins and mistakes and so seeks to repent in front of God. And if your enemies are forgiven by God but you still have hate in your heart for them, any possible reconciliation between you and your enemy may still be problematic, needlessly so in fact once God has given green lights for your enemy.
In addition, conversely, it is likely that you have sinned too as you engaged in the persecution of your enemy. Consequently, it is likely that you too need to ask for forgiveness with the Lord, and if your enemy still has hate in his heart for you after you’ve been pardoned by God, reconciliation may be a problem from this other perspective.
Also if you pray for your enemies what you do with respect to yourself is to psychologically help prepare yourself to be ready to forgive your enemy once this should become a desirable thing to do. By praying for your enemies you basically admit to yourself and God that you are ready or at least willing (under certain circumstances perhaps) to forgive them.
So basically hate causes strive and makes reconciliation between inimical camps difficult. As you know, hate is product of the ego and so it is indeed the ego that is the ultimate obstacle for the establishing of harmony and unity in humanity.
Once we have mastered the hate and fear in our hearts and hopefully replaced it by love, then there stands nothing in the way of our spiritual ascension to ultimately join God and assume our rightful place next to him in the heavenly Kingdom.
Love enables the fulfillment of this destiny whereas hate and fear, on the basis of ignorance, try to block it… with a passion.