He assumes it was starving because it was much smaller than the litter it was born into. You then gave it to a mother with a smaller litter and that litter outgrew it.
So are you saying that at the time you put it down it was the same size as its mothers next litter? That's insanely slow growth. How could you possibly say "It ate very well". You should have culled it. Rhasputin gave you good advice that you asked for. You ignored it and the ASF suffered as a result.