Apparently, one has to do something brave in order to receive the roar for the first time - Kion stuck up for Bunga against the hyenas. I have made up this small fan theory where Scar actually earns his Scar while defending his big brother from something, and carries a nickname as a medal for strength [or a type of a super hero code name] - but then, when they are adults, he grows jealous and does what he does. It just popped into my head this instant, and I think it's a fun theory to work with.
All in all, I wonder if Scar's going to play a bigger role in the series, or is he going to be a cave painting for the rest of his existence. In one scene, we see his painted face looking all ominous, but it doesn't play a big role because Kion didn't get jealous or bitter with Kiara being a queen. The series is going to have that 'I just can't wait to be Quen' episode for Kiara, and the cover of the referenced book has Kion looking a bit... unhappy, so I wonder if it's going to get worked on a little bit more. And somewhere they said we will get to see both Scar and Sarabi as ghosties.
I'd really be happy if we at least got one animation of Scar during the series but... expecting things only brings disappointment.