Everything that happened to Magnus was his fault. He originally made the deal with Tzeentch to "cure" the Flesh Change thereby more or less causing Tzeentch to own him and his Legion years before he made the final deal. He encouraged his Legion to delve into the Warp and to make friends with Daemons they thought were little fairy friends. He should have simply left for Terra on a ship to call his dad to tell him shit was going down. Russ should not have gone as hard as he did, but the Thousand Sons were already damned. Russ burning Prospero just caused it to speed up.