To be fair, the insane man still have a point.
That said, none of that justify murdering entire cities and inhumane experimentation.
So he is still the bad guy, but he does have a point that the MC has some big skeletons in his closet.
Human experimentation is a terrible thing unchecked, but it is also sometimes required to progress.
Especially in medicine and probably healing magic too.
MC´s other "sins" are pretty deplorable too, but again.... does not justify murdering whole cities over.
pretty good villain backstory, even if it comes down to TL : DR
"i got but hurt over what you did in the past and twisted it out of proportions"