At soft play today, two brothers were fighting. I recognise them from toddler group and have chatted to their mum on many occasions - they are aged 5 and 3. She was busy chatting so on the way to take DD to the toilet I mentioned to her that they were fighting in there and she just shrugged it off as 'what boys do.'
When we went back into the play area a few minutes later, the older brother had the hysterical younger brother pinned to the floor and was alternating between pulling his head up by the hair and smashing it on the floor and whacking him over the back of the head with a big soft play shape. I told him to get off his brother and that he was hurting him. He smirked and carried on, I looked over to his mum who shrugged and rolled her eyes. I couldn't listen to the poor little boy screaming anymore so I removed the soft play shape from the older boys hands as he raised it to hit him again and sternly told him to get off and leave his brother alone.
He went running off to his mum and she came over and said I'd understand more about boys when baby DS is older (my other DC are girls) and that the boys are capable of sorting disagreements out between themselves without my intervention 
I know it wasn't really my place but the boy was being so spiteful and his poor brother looked terrified. Was I BU to ignore the mum and intervene?