That didn't stop Ruin from trying to, well, ruin everything. Plus, the Almighty legitmately fears that Odium (or, to be fair, whoever else "the enemy" might refer to) will destroy everything. If even one of the Shardholders well and truly fears it, to the point that he's going to all this effort to stop it from happening, that has to account for something.
Also, hatred can be blind. Perhaps Odium is simply acting on his nature, and can't stop and think what consequences killing everyone and destroying everything would have for him.