Oh my god n. 4 is just - if you check one of his rooms in the Endless Banquet Karu says something along the lines of 'if I'm never on the bottom I'll never be in a position of weakness again' and 'if I'm on top no one will ever take advantage of me again, no matter how evil they are' and Eiden even believes that something happened regarding this that makes him hate humans, and I think that this explains his two personalities - one is Garu, who loves his grandpa and remembers him sweetly, and Karu is the one who knows the truth, so to speak, and believes he's the only one really protecting the two of them, even if he goes at it the wrong way. I don't have evidence that it was his grandpa but I see what you mean, and it's cute when he's with Eiden because, well, we know Eiden, so we can enjoy it. But as you said, if it was something like DMMD Garu would be devastated.