You did the right thing for Lion all the way down the line. You gave him the best vet care, you chose not to put him through more when he'd had enough and you were with him in his final moments, during his passing to the RB and even afterwards, holding him in your arms. There's nothing to regret and only good memories will remain after the initial pain wears off. Lion will always be but a thought away to comfort you and let you know that all is well. A candle burns for you both.