We have a lot of sympathy for that perspective.
However, here's my response to that input:
We understand that. Have some sympathy for that.
However, have been moved in our spirits that this is war. And that King David--a man after God's own heart--prayed some fierce prayers against his enemies. And, we have felt increasingly in our spirits to do the same thing on occasion. We do ask God--according to, subject to His preferences and priorities, of course.
Onward Christian Soldiers is not about wimps or wimpy responses to evil. Nor even about a quiet, gentle response to evil.
Though in general, we can have a ton of sympathy for your stance. So, feel free to leave those parts out!
And, we believe that some of these evil doers have sold out their hearts in blood vows to satan. We are skeptical that there is any hope for those sorts of people and feel led to pray firm prayers clipping their wings or worse.
YMMV, of course.
Mercy to the cruel
Is cruelty to the innocent
Plus the cruel need punishment for rectification and hopefully have a change of heart. So in that very real sense, punishment IS mercy.