Hi jesyka. Sorry for the loss of your "boy". It must be difficult to let go after so many years of joy together.
From what I have read survivor's guilt is common whether in losing person or pet. You did the right thing in my eyes. You have him a chance to live a full life. When their lifestyle disappeared then you gave them a painfree departure.
They even mewed to you good bye. I am sorry for the loss of your friend.