Don't feel guilty - most animals prefer to die alone if they have the ability to do it - domesticated and wild ones alike. I've had 2 cats pass on at home, and they both picked out hiding places in the house to do it. The kids also had rabbits outside in a hutch when they were small, and the last survivor actually chewed thru the wire of the hutch to get out to go off and die. Fortunately I found him before the kids did.
Salem is at peace now and would not want you to feel guilty. I'm sure she knew you were trying to help her and loves you for all of your love and companionship over the years.

RIP Salem