this may be speculation but I think that the current Ignis is not as bad as they MC describes but rather something in the future occurs which causes him to change into the tyrant she describes
@proslayer99 well, we only know that his fiancee will fall in love with his half-brother, who was hidden away. It is kind of messed up, as appearance of another possible heir, who has support of grand duke's daughter, may very well lead to chaos and death. Considering Ignis has somewhat complicated relationship with his father(who supposedly prefers Letio over Ignis), this stress piling up could have caused him to become more cruel.