The following is an old Gaelic blessing:

May those who love us, love us,
And those who don't love us,
May God turn their hearts,
And if he doesn't turn their hearts,
May he turn their ankles
So we'll know them by their limping.

I think it's safe to say that we all struggle with our attitude toward our enemies.  We know what Jesus said:

"Love your enemies,
bless those who curse you,
do good to those who hate you,
and pray for those who spitefully
use you and persecute you."
(Matthew. 5:44)

(and I don't think the prayer above was what Jesus had in mind!)

But there are, throughout scripture, a number of examples of righteous men calling God to act against their enemies.  The Psalms are filled with such pleas.  And even in the New Testament,

"...The souls of those who had been slain for
the word of God.....cried with a loud voice,
saying, 'How long, O Lord, holy and true,
until You judge and avenge our blood on
those who dwell on the earth?' " (Rev. 6:9-10)

When we see great evil being carried out, I think there should be a part of us that cries out for justice.  We want to see God avenge those who defiantly oppose Him (and we know that He will).

But, as difficult as it may be at times, we also pray that God will help us to respond personally to those who are our "enemies" in such a way that they may eventually be led to be our friends and, more importantly, friends of God.