That's because the general idea of reducing a population in order to save them (or their environment) _is_ rational, as has already been pointed out in this thread. It's the over-generalization and scope that turns it into madness.
I recall reading about the 'advantageous' side effect of wars of reducing populations. There's definitely precedence. I think it's more a question of morality than rationality.
I.e. other methods of regulating population growth may be less questionable.
But, that's not how culling works. You don't cull once and then walk away. You cull periodically. That's what hunting seasons in North America actually are. There's a reason that we have those seasons and the limitations on the number of animals killed.
The notion that you can cull once and walk away is completely irrational. And, frankly, wars have never really reduced populations. At least, not in the longer term and even in the short term. Good grief, we've killed more people in wars in the last hundred years than in the past ten thousand and yet we've managed to increase our population several times in the same period.
But, again, do we really need to spell it out in the movie? "Hey folks, I'm amassing god-like powers. I am now literally the closest thing to a god in the universe. What am I going to do with these cosmic powers? I'm going to wipe out half the universe, resulting in massive extinctions throughout the universe as worlds completely collapse in the aftermath and then I'm going to rest beside a lake".
Do we literally need someone in the movie to turn to the camera and say, "Gee, what a nutter"?