So i was playing control priest vs Blood DK. He discovered Rivendare Warrider via E.T.C. He summoned it, I killed it. Next turn he summoned the lifesteal one and I silenced it before killing it. Also, he didn't copied it or anything before killing it. 15 mins later he kills his own rush raider with corpse explosion and the win condition triggered. Is this supposed to happen?
I suspect that the deathrattle says if all raiders died this game, regardless if those were silenced or not before killing them, but I am not sure.
As Satoshi correctly pointed out, silencing one of the first minions doesn't do anything. Silence removes that minion's card text, but it's still that minion. When the final one dies, its Deathrattle triggers, which checks the graveyard and the game sees that all other Horsemen have died. It's rather frustrating, but it's not a bug.
And yes, this is why players tend to kill off their final Horseman in their own turn. To prevent that one getting silenced.