Acquaintance's kid was the worst case of sore loser I've ever seen. Her parents would always let her win. Eventually she refused to play games with anyone but her parents because she was too afraid of losing. I'd visit and she wouldn't even want to play fully random games like uno.
Kid's been in and out of psych wards throughout her teenage years. It's possible that underlying mental health predisposition caused her behavior but I think giving in to it only hardened those pathways in her head over the years. It's vital to learn how to lose and still enjoy yourself.