I think you are fine. Yeah it may suck for the player for losing 2 characters, but when you base things on the luck of the roll, that can happen sometimes. As for the encounter itself, I think you did fine.
You know your player/characters capabilities than the others on this board, and obviously they did well since they only lost one character. Those who are saying you were overpowered, I fail to see their point, its within every DMs right to dishout difficult encounters once in a while, its not like you do it every time! If you did, you'd not be posting here with consern over killing a player.
I myself have challenged my players with encounters that were 5 pts above their ave CR all the time. And it would occasionally result in character deaths. But that happens and is part of the game. If the player was REALLY not wanting to lose the character, something can always be done to get the character back.
K Koie