It is the choirmaster or mistress' job to tell a singer as kindly as possible that their voice is no longer suitable.
Please leave well alone, if you want to stay friends with the person in question.
If the friend is actually yourself, then ask the leader of the choir whether he or she agrees with your own feeling that your voice is not up to it any longer. It comes to us all, and by broaching the matter yourself, you will make a difficult discussion easier for both of you.
Likewise the cat; I personally prefer to have a beloved pet put to sleep rather than subject it to treatment that at best will only prolong its life a little longer.
But deciding when the time has come, is as anyone who has ever had a pet knows, a horrible decision to make.
If you are asked directly for your opinion, you might feel able to say gently, "Perhaps the time has come where puss is no longer enjoying life." but it is not your opinion or mine that counts here.
Hiding the car keys is a coward's way out, only of use, if we are talking about a host preventing a guest who has had too much to drink from driving home.
If it is time to tell your spouse or another close relative that they are no longer safe drivers, this has, in my experience, to be said straight out, again as kindly as possible.
The person concerned will be hurt - no doubt about that and if he or she takes no notice there is little you can do, except refuse to get into the car when they are driving.