@Pax_Cruento well he bought into the idea he was dead weight, something reinforced by the fact on paper he should be just that compared to the rest of the party. in practice this is hardly the case, and yes he should have known better, especially since the sage is very much an asshole and he would know his sister better than to think it's something she'd agree on. having said that though, from his perspective this isn't a participation game, it's a war against the friggin' demon lord. if he comes up lacking against the stuff they fight at all then they'd have to protect him and in that regard it makes him a liability. his sister being able to take out one of the generals without him cements this in his mind. (granted we know this isn't the case and frankly without him doing all the heavy lifting the party is coming up short and barely scraping by, and they make it known how upset they are with the sage, but red has no way of knowing this) so, yea, it's kinda stupid that he just caved, he should have contested the sage's assertions more or asked the party as a whole how they felt, but it wasn't a baseless decision on his part, there was some logic to it. (just that logic only holds up on paper)
for what it's worth though, i'd say this worked out in his favor considering the presence of lit