Raelag is a very interesting character because he has undergone many transformations in his life. If you recall his demonic name (or rather, its English version - Agrael), you can easily see that Agrael is a light elf name. Just like Dirael, Wingael, Vinrael, Arniel. The translation "Agrail" was probably chosen only because "Raelag" doesn't sound very good. I suspect that Agrael was originally a light elf. Then he became a dark elf, Raelag. Then, for some reason, he turned to Urgash and became Agrael again. However, he didn't become a full-fledged demon (remember: in Heroes 5, the Griffin Heart, which affected all demons, had no effect on him). Tieru cleansed Agrael of demonic corruption, and he transformed back into a dark elf, taking his former name - Raelag.
well, let's assume that's the case. Then, probably, during his first time as a dark elf, Raelag was someone from the Clan of the Stern Hearts. Then it's strange that, as soon as he became one again, he immediately went to destroy them.