heresy question

Hey guys. I know I'll definitely get flamed for this one, but I was thinking, why is heresy sinful in the first place? Obviously, I know the Church condemns it and its a grave sin because it's falsehood. Certain heresies like Jansenism make sense as it causes people to despair and could affect their salvation. I know God is truth, so it would make sense to not perpetuate lies. BTW, I'm not looking for an excuse to preach heresy obviously, but I was just thinking this to myself the other day.

I guess I answered my question? Idk. Do y'all get what I'm trying to say or am I just dumb?