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It seems that way. Agrail couldn't prevent the coming of the Dark Messiah, he could only delay it. On the other hand, it was thanks to this delay that Kha-Belekh was defeated. It is unknown what would have happened if Sareta had been raised by her father.
Remember the last video, about the murder of Biara. Kujin said that Biara supposedly killed Isabel's husband. And then Isabel says to Biara, "My husband, my country, you will have to answer for many things." I was waiting for Biara to tell Isabel the truth, and for Isabel to hate Railag. But, alas, a happy ending with wild laughter prevented that.
In short, Railag, it seems, lied to Isabel. He's not as simple as he seems. But then again, that's what you'd expect from a dark elf.