He wanted to make sure End didn't die so he could make him the "undead lord." If End died, his legacy would have been a lot harder to live through, wouldn't it? From our perspective, Necromancer Horus is the MC's captive, but from his perspective End is his greatest creation. Even if End fought Horus it wouldn't make much of a difference, End is a prodigious ghoul but ultimately he is a lowly ghoul, it would have been frankly stupid to summon him. I don't think this spoils too much but it is a spoiler nonetheless.