I feel like part of why Armand hates Claudia is that he's jealous of her. I feel like a part of Armand wishes that he was turned as young as Claudia was.
Armand was in a brothel until he was 15, when he was bought out and subsequently groomed and forced into prostitution by Marius. We saw Armand romanticize their relationship in his s2e4 monologue, he clearly views Marius as this white knight savior that truly did "care" about him. And after being turned and losing Marius, Armand hasn't had anyone who really "cared" about him. He started a relationship with Lestat but that quickly fizzled out. He tried to start one with Louis but Louis really just wanted something casual.
And I think Armand believes that he would be more desired if he was physically a teenager like Claudia. If he was still the same age as when Marius groomed him into a relationship. If he was still the same age as when everyone in the brothel and within Marius' social circles wanted him.
It's why Armand is so harsh towards Claudia when she says that playing Lulu feels humiliating. After all, the play was a hit. Claudia was able to make the audience love her by physically looking like and acting like a child. Claudia has the one thing that Armand wants the most but will never have and he hates that she's being so "ungrateful" about it.
And I think this jealousy is why Armand repeatedly talks about how damaging it supposedly is to be turned as young as Claudia was, saying that her mind will eventually break or that she'll throw herself into a fire one day.
Armand needs this to be true because if there is a huge downside to being turned young, then there's nothing for him to really be envious about.
And I also think Armand needs this to be true because it's what Marius told him. It was Marius' supposed reason for not turning Armand sooner. (s2e4: "his reluctance to share the dark gift, knowing what it would do to his beloved Amadeo")
And if it turns out there is no real risk to being turned young, then I think a part of Armand will start to resent Marius for not turning him as young as Claudia. Or he'll start to question if Marius knew that this risk didn't exist all along, start to question if Marius really "cared"/"loved" him as much as Armand thought he did.
And if that happens, Armand will truly have nothing left. To Armand, Marius is like the one good thing that happened to him. In fact, I bet one reason that Armand has stayed alive this long is that he didn't want Marius' "sacrifice" (i.e. being executed by the Roman coven for turning someone with an illness, so essentially dying so Armand could live) to have been for nothing.