@Benis Don't forget that this is the MC's revenge... To him, it's not much different in another universe or not. Besides, why did mc has to suffer so much in the past for less than one reason that he didn't understand much.. That's why he thought of something for them to deserve more than they already are, although as you say it is cruel but think back, why did they have to mess with his old grandma and his little brother who is innocent..why can't they leave em be?? Why did they have to suffer too??!
Enjoys action, comedy, slice of life..
Spoiler!
Love bromance