Blame those, who started the war, instead of those, who ended it.
Defenders can use anything, including weapons of mass destruction, to defend themselves. Attacking to with intent to kill even one person is crime.
Not according to the Geneva convention. Targeting civilians is a war crime, regardless of who does it.
In many wars, both sides claim to only defend themselves, often both sides even claim to have been attacked first. Just look at the last few wars fought by the US for example. Under such a simplistic moral compass as you gave, they'd both feel justified to do anything.
> Attacking to with intent to kill even one person is crime.
Dropping an atomic bomb on a civilian center is attacking with intent to kill.
It is just not so simple.
However, this should not alter your moral compass. Those, who defend themselves, have moral rights to wipe the attackers or invaders.