No, I wouldn't call him Yandere... he's pretty normal when it comes to how he acts, he's just a chill shonen archetype dude who just happens to go crazy in his demon form, what I mean is that his personality is pretty even through out, even when he hulks out he has some sort of human Melodias reasoning and comes back to his senses. So, nah, not yandere.
Now to answer the question, I think these "dere" characters are boring and one dimensional, how could they not be when you are putting them in boxes from the start? Granted, there are some GREAT "dere" characters out there like tsundere Asuka from Evangelion and, believe it or not, yandere Littlefinger from Game of Thrones! Most of the time, however, they are just predictable as in you already know what's gonna happen (tsundere girl is mean, warms up to boy, falls in love, "BAKA!!!", yandere girl is cute, she is in love with boy, boy likes other girl, yandere girl kills girl, she kills boy/self, or ends up with boy) you know what I mean?
Actually, Littlefinger is a fucking great yandere now that I think about it... he's all loving and sweet to Catelin and Sansa but he's malicious and nefarious to everyone else who is in his way... and not to get into spoilers but if you watch the show you know what I know and you know he got his just desserts. ANYWAY, it goes to show that you can use "dere" characteristics as BASES for characters but not have them be completely ruled by their "dere" aspects. Littlefinger is a great example for a yandere character done well, he isn't defined by "dere" but he's well rounded and a great villain because he is also human with human emotions and human desires. Pretty much that's the thing, you have to make characters human and then break them to your mold instead of having them be caricatures of humans and then build them up to humanity, you can't bake a cake backwards.