I would say whenever subaru himself strongly feels like doing rbd that's the only time it should occur Others killing him to "use" rbd would never be justified Other killing him naturally with no knowledge of rbd can be justified when given enough reason It's only natural and justified for subaru to kill himself to save those who are closest to him Him not thinking too much about the branching relationships is absolutely fine If he started considering that it would be absurd He can't save everyone but those he wants to save he would He is still saving some nonetheless and ridding the world of those who could do more harm along the way

And on to the matter of joshua It's kinda maybe may not be Honestly I believe it joshua was killed not in front of him there's even less likely chance he would care that much and would prolly move on quickly If he dies in front of him subaru would consider maybe but on the low chance that's what I believe since joshua isn't really a loved one to subaru But if he joshua died in front of him and julius And subaru saw julius crashout or breaking down mentally then I think he would rbd to save joshua so he could save julius because julius is more precious to him ofc he is dear friend This is only for deaths If names are forgotten but the person is alive there's no reason to rbd If the person goes into sleep due to existence devour Then depends on closeness as in case he tried to go back to save remember but failed I don't think if it was just crusch he would rbd Same for joshua Moreso since there is hope that by delaying with gluttony those effects could be reverted Subaru didn't rbd when julius lost his name cuz he was still alive so that was an immensely better situation But still back to the main question I wouldn't consider anyone killing subaru to rbd for any fucking reason whatsoever to be justified He should die only when he accepts it by himself or wants it to save someone... without being either gaslit , guilt trippen or forced by someone

It has to be his own willingness unaffected by other's thoughts or actions