I actually think End probably had the opposite problem. If there's Positive Energy and it can build up in someone naturally, why can't the same happen with Negative Energy? We see what appears to be some sort of Possessed Child in this chapter.
Keep in mind that Senri and End's conditions aren't perfectly the same. Senri was weak and bedridden, but she never mentions being in physical pain, while End was apparently in Agony 24/7 due to his 'disease'. It's possible that End had the same condition but just more of it so he had worse symptoms, but I think its more likely that there's a Negative Energy that Necromancer's draw their power from and that was what he had, either naturally or because someone was trying to kill him to take his inheritance.
This could also explain why he got pulled into his new Body. Perhaps when he died, the Negative Energy he had drifted back to the Negative Energy Miasma or whatever it is that Necromancer's draw power from, and it pulled his Soul with it. So then when Necrodude drew from this pool to crate his new Undead, it pulled End's Soul with it.
This would also explain why End never got picked up by the Holy Order when he had it, even though the first chapter indicates he was getting visits from White Mages. I mean if Senri, who was clearly some Peasant Girl based on the garb of her parents in this chapter could get someone to figure out what was going on with her then surely mister Noble who gets top White Mages would get found out too. Though I suppose that could be explained by an angry claimant bribing the White Mages to not report it.