
In the case of furniture made with body parts of the Primals, I'd say that's impossible physically. As far as I recall, the Primals vanish after they die, so there would be no body left to butcher.
However, you can still have that furniture ICly. The justification for their existence is that they aren't made with a piece of the Primal. Maybe there's a carpenter, sculptor or what have you out there with a weird sense of aesthetics that uses the Primals as his inspiration.
On the matter of slaying of Primals IC, I'd say that's possible but probably not a very good idea. Later on in the storyline, you meet a group of adventurers who slayed Titan. They are quite famous in-universe, so slaying a Primal is no small feat. What's more, in the main storyline you, the player, are considered quite an extraordinary fellow for having defeated Ifrit. There's also the matter of tempering Zarek mentioned (which, by the way, to be tempered the Primal has to directly try to do so: it's not enough to be in their pressence). Only users of the Echo or those tempered by another Primal are immune to it.
Unless it's a a cover operation (meaning maybe no one but your group knows about it), I would suggest against Primal slaying in-character. Even if the lore supports the idea of slaying the Primals multiple times, there's the implication that you should become pretty damn famous for that.
However, you can still have that furniture ICly. The justification for their existence is that they aren't made with a piece of the Primal. Maybe there's a carpenter, sculptor or what have you out there with a weird sense of aesthetics that uses the Primals as his inspiration.
On the matter of slaying of Primals IC, I'd say that's possible but probably not a very good idea. Later on in the storyline, you meet a group of adventurers who slayed Titan. They are quite famous in-universe, so slaying a Primal is no small feat. What's more, in the main storyline you, the player, are considered quite an extraordinary fellow for having defeated Ifrit. There's also the matter of tempering Zarek mentioned (which, by the way, to be tempered the Primal has to directly try to do so: it's not enough to be in their pressence). Only users of the Echo or those tempered by another Primal are immune to it.
Unless it's a a cover operation (meaning maybe no one but your group knows about it), I would suggest against Primal slaying in-character. Even if the lore supports the idea of slaying the Primals multiple times, there's the implication that you should become pretty damn famous for that.