Retaliation begets more and more... on smaller scales look at Ireland, Israel/Palestine;
These things never go away, they become entranched and fester like infections, turning all they touch into sewers.

Pray for the victims.
Pray for the survivors.
Pray for their loved ones and their families.
Pray for the hate and revenge in the hearts of all to be healed (including the perpetrators).

When the hate stops, then the killing will stop.

Retaliation does not stop hate... It feeds it.