We know what the words the real stormfather needed were.
Life before Death, Strength before Weakness, Journey before destination.
So either the Stormfather is evil, or someone else is talking to Gavilar.
Plus, the Stormfather lied. We know that going against the intent of a Shard is lethal to the Vessel, why should it be differ for fractions of a Shard? The Stormfather is the biggest fraction of Honors investiture, and I don’t think extensive plotting and lying is not very honorable (Although Syl, imitating an voidspren, and Pattern, lying to the Honorspren, have both lied). We can extrapolate that this change in personality is not bc of Gavilar, because the voice implies that it was plotting before it decided to chose Gavilar.