Blood Flowers

After successfully having killed the entire world, a swordsman gets reincarnated into another world.

Now having been filled with the regret of his own consequences and actions, he puts behind the life of violence in the pursuit of flowers.

Instead of taking life, he will create life. For every person he killed, he would have to plant a flower. This was his only means of atonement. However, in this day and age, to never pick up a blade, is to allow others to do it for you.

And when they fail, you ultimately lose them. You may end up being the one that caused their death due to your inaction. Thus, his atonement may very well be for naught. Especially when the consequences of violence consumes him again.