The prophets of Israel are not like the prophets of the nations of the world. Balaam (a gentile prophet, see Numbers 22:2-24:25) sought to uproot Israel over nothing, but the prophets of Israel mourn over the retribution that befalls the nations.
I make no comment here as to whether this is good or bad (Rashi makes no comment to this effect either, though the implication is that Rashi is not criticizing the prophet Isaiah), but it seems to be the undying trait of the Jew to love and feel compassion and act mercifully toward those who would kill us given the chance. Let no one think that ours is the first generation to behave this way.
Let us, better, grieve over calamities striking our evil enemies, than to grieve over even one Jewish child pricking his little finger.
We of course are obligated to protect ourselves from someone coming to harm us. Does not mean we cannot pity the condition that makes these humans act in such violent ways. The trick is not to put pity before protection. Protect first, pity later.