FWIW, I have a gelding by Devil His Due (Halo in his pedigree). My horse is really sweet and kind…until he is NOT. He can be quite aggressive, and he can switch from very sweet to downright evil, and then right back again…quick as can be. He despises small animals - dogs, cats, etc. and will go after them if not watched/corrected.
I’ve owned him for 4 1/2 years, love him, and have learned how he ticks. We make it work, but I’ve had to put a lot of effort into figuring him out. Most people don’t believe me about his “evil” side until they see it. He is sometimes so quiet and kind that a child could ride him. Other times, not at all. I can w/t/c on him bareback (sometimes…other times, I can barely walk him to the ring or paddock). He’s very interesting and has taught me a LOT about working with a difficult horse. He’s also incredibly intelligent, so when he uses that intelligence for good, he is the best horse I’ve ever ridden.
He would not be a safe or fun choice for the average child or amateur, as you have to be on your toes 100% of the time. I’ve rearranged significant aspects of my life to manage him properly (he has unique riding schedule needs, particularly in winter).
I’ve done a lot of looking into his pedigree, and have found that many people make similar comments about horses from the Halo line. One person described it best in saying that they have “light switch” personalities - fine, fine, fine, but if you “cross” them, WATCH OUT! Half the time, I don’t even know what I’ve done to suddenly anger him. But I always know it is a possibility that he will become suddenly angered (often by something as simple as a cat meowing and walking near him).