No one should shame you for not staying for euthanasia… shame on them! We all (animals as well as people) process things differently and as @Djones noted, there is no one right way. Panda knew you loved him.
I have always stayed. I have never had a vet ask me to leave. Maybe because at that moment I am sad… but calm. I need to be calm for that dog/cat/horse. They need to feel relief and release. Horses that I have brought into this world… I need to see them out of this world as well with the same love I used to greet them. Dogs that came to me as a happy, trusting puppy… I will soothe them One Last Time and send them on.
I have even helped other friends who cannot stay… and even taken their dogs to the vet for them after a sweet and private farewell.
I like to think that I make it easier for friends and their pets. And after all is done I can go home and finally be sad and have that glass of Pinot Grief.
The worst euthanasia I experienced was a mare brought in to the animal science program I was in to be euthed - she reacted violently and flipped herself over backwards.
The other extreme… my 16 year old border terrier was in discomfort in May 2020. COVID restrictions in full force. My usual vet clinic was closed. I took him to a big emergency clinic and had to hand him off to a tech in the parking lot… he was a sweet little guy but was very worried as she took him away. X-rays and tests revealed cancer growing in his abdomen. I took him home and he had a lovely time doing his favorite things for a couple of weeks until he was obviously failing and had quit eating.
I was not going to take him back and just hand him off in the parking lot. I called my equine vet as there was no home service I could find… and she said that of course she would come and help.
It was a lovely sunny day. Tug wandered around stiffly, carefully sniffing all the sniffs. I settled him on the tailgate of the truck where he used to easily spring up and lay to survey his kingdom. He got bacon - that he would eat with enthusiasm. The other terrier (Tally) jumped up there with us… and there we sat. When the vet came she joined us - Tug happily greeted his friend and we chatted about dogs and how awesome Tug was. He slowly nodded off between us in the sunshine and the vet gave him the sedative… so just a deeper sleep than the nap he was already taking. With the final shot he simply drifted away and left that tired, painful old body behind him. And then… the vet and I watched Tally jump off the tailgate and spin and bark, do her happy, playful dance and bounce off down the driveway past the paddocks. I like to think she was saying her final goodbyes as Tug danced away with her.
The vet refused to charge me anything for that visit. She just said she owed Tug a favor.
Tally and I buried him under a magnificent cedar tree, knowing that whoever may own the farm in the future will respect that tree.