A reasonable actor wouldn't counter attack a first strike because annihilation of mankind is not an option.
If the options are: yield the planet over to some nuclear hardened species to chill out and recover for a few hundred thousand years while other novel (radiation hardened) lifeforms develop, or yield the planet over to the people who have demonstrated willingness to absolutely obliterate their enemies and will likely continue initiating mass extinction events far into the future… I certainly don’t think one option is 100% off the table.
Interestingly, those radiation hardened life forms might actually be better suited to be earth-seed than us — space is an unfriendly environment for human kind, making us ill suited to be the planet’s intergalactic ambassadors.
(Now I want to read some sci fi book where it turns out all the intergalactic species come from planets where the original intelligent inhabitants blew themselves to smithereens and created a nuclear winter, causing the next intelligence to develop a deep rooted desire for world peace and scientific exploration…)
If you are christian you believe mankind lives on in a happy afterlife event, nobody really dies. So no biggie if you shoot back.
If you are atheist, it's not clear why you should care about the continued existence of a species evil enough to murder your friends and family. Seeking justice for them makes more sense. At any rate it's not reasonable for an atheist to believe our species would live forever. We were going to eventually die anyway.
And justice is meaningless if no one is left.
People find meaning when they are alive. The person firing the missle- being alive- has as much as anyone else.