It seems this one is before Fotd, and honestly the author writing style a bit different and much more mature at Fotd, so
Why ML changed this life time ? MC also confused, but mostly it's bc MC is unlike the previous life, in previous life he's already sold to a pedo by his mom, didn't have a chance to study bc mom like to abuse MC and prefer to sell him, doesn't have manner, and even when he's trying so hard to get recognized by ML it failed, everyone didn't like him too, probably bc they think MC 'dirty'. But now, bc MC has the power to control others (he can command his mom to do sth), so the mom couldn't do anything to MC, and bc MC has knowledge related to heart surgical (to the point of genius) etc etc plus he looks beautiful (unlike his previous life where he's malnutrition or sth), so
Ah, also MC super indifferent to everyone and only close w/ the sister, he only believes his sister . . . .
As for why MC forgive ML, bc MC thought ML (and everyone) is different, and unlike the previous life, actually i don't understand why MC didn't do anything to ML bc of previous life though . . . . close to the end, ML couldn't walk after an accident bc he saved MC's life, that's maybe why MC also decided to forgive ML orz