I've only had ponies taken away which involves winching - Douglasbrae who come for them are always very respectful and suggest I go somewhere while they do that, but it's not a nice sight. They've never let me see any other stock on the wagon, it's all tarped over and secured throughout.
A couple of times have not been possible to go away and not watch - when my first old mare age 26 was pts it was a local farmer that came with a JCB to bury her (it was permitted then) and he lifted her on the bucket and dropped her in the grave he'd dug. I can tell you that was no easier to witness
In April I had the worst ever experience of a pony dying - she was 4, bred by myself, a favourite, and from the accident at 10-11am she had a chance of recovery so I sat in the field for hours until it became obvious her time was over. I called the vets back at 3, they came at 5
and by that time it was too late for Douglasbrae to come out until 830 next morning so a friend had to drag my beautiful girl up the field with a tractor during the evening rush hour on the main road that passes the field length
I had to cover her in the yard overnight, and then a neighbour had to come out with a forklift to load her on the lorry next morning (yep, rush hour again) cos the wagon couldn't get in my drive to winch her
Horrendous and I had to be behind the tractor to open and close gates so I didn't leave next morning when they suggested it as I had already seen the worst. It is just heartbreaking..
I've also had to bury 4 cats and a kitten, and an aborted 7 month foal foetus - so I reckon I am pretty capable when needed and just fall apart afterwards, trying not to upset the professional knacker men
I'm so sorry you had to see the other stock - in answer to your question (with apologies for venting my own story) I would leave the barrow with him another time, these guys do understand and I find would far rather get on with it without owner emotions which they find more difficult to deal with..