The genius of Terry Pratchett.
In this quotation from Unseen Academicals – about sports, ugh – my favourite Discworld character, Havelock Vetinari, explains his first encounter with evil and what that meant to him in terms of moral engagement.
The Patrician of Ankh-Morpork: Lord Vetinari
The Patrician took a sip of his beer. “I have told this to few people, gentlemen, and I suspect I never will again, but one day when I was a young boy on holiday in Uberwald I was walking along the bank of a stream when I saw a mother otter with her cubs. A very endearing sight, I’m sure you will agree, and even as I watched, the mother otter dived into the water and came up with a plump salmon, which she subdued and dragged on to a half-submerged log. As she ate it, while of course it was still alive, the body split and I remember to this day the sweet pinkness of its roes as they spilled out, much to the delight of the baby otters who scrambled over themselves to feed on the delicacy. One of nature’s wonders, gentlemen: mother and children dining upon mother and children. And that’s when I first learned about evil. It is built into the very nature of the universe. Every world spins in pain. If there is any kind of supreme being, I told myself, it is up to all of us to become his moral superior.”
The two wizards exchanged a glance. Vetinari was staring into the depths of his beer mug and they were glad that they did not know what he saw in there.
Schopenhauer certainly has a challenger for the most eloquent pessimist.
The Indelible Stamp 
His statement doesn’t seem pessimistic at all to me. On the contrary, it is, as you’d say, a very engaging one. It tells us that we can be better, and that we must.
They (pessimism and engagement to do beter) are not mutually exclusive. Pessimism, as a tradition, as as Joshua Foa Dienstag and others defend it, is not about resignation or apathy. This is the way it ought to be understood in order for it to be useful.
Luckily, there is no god so we have nothing to worry about in the moral superiority department!
I must say this quote set me back on my heels. I haven’t heard a quote about religion make me think so much in a long time(and this from a fantasy novel). The challenge to be morally superior to a god. Assuming a god is omnipotent, then he obviously allows evil in the world and by allowing it, he causes it because he could just not allow evil. Christianity sees this evil world god created as a world that God intended for us to rise above for our salvation, whereas Vetinari sees this evil world and concludes God is evil and Vetinari challenges us to rise above God. That is a big difference. I’d rather be asked by God to beat an evil world than try to beat an evil god. God is good and in this world he gives us a chance at moral salvation or god is evil for making this evil world and we must try to be morally superior to him. Which world would you rather live in? Hint: either way, an omnipotent God always gets his way. Peace.
Nature is red in tooth and claw :-)