A lot of 6 year olds are more than capable of getting the facts right when they retell something that happened, just as some 6 year olds can be very horrid, sly, manipulative etc. And some are horrible intentionally as they like to get a reaction out of people. I once found out about some bullying going on in my class of Year 1 children by 2 boys against a girl with special needs (in the playground when I wasn't on playground duty) because another little girl went home and told her mum, so the mum came and told me. The girl who told her mum was very bright, articulate and empathetic, and I knew she was telling the truth. Plus, the girl who was being picked on told her mum too, who came to me in tears because of how these boys were making her daughter feel. They were quite capable of doing it purposefully and slyly - making sure they were out of sight and earshot of teachers, and picking on her because they knew she wouldn't be able to stand up for herself.
I once did some supply teaching in a Nursery class for a few days and had a little girl, 'Molly' in the class who was best friends with 'Sarah.' Molly tried to have complete control over Sarah all the time. When it was time to sit on the carpet, if Sarah wanted to sit next to Molly she would be told, 'Eurgh, go away! I don't want to sit next to you!' But then, as soon as she went to sit somewhere else Molly would say 'I'm only joking, you can sit next to me!' and Sarah would immediately go back to her. Or, if she ever did try to sit next to someone else first she would be told, 'You're not my friend if you don't sit next to me!' So Sarah would meekly go back to Molly. Then, Molly would sit and do things like pull Sarah's hair, pinch her, whisper in her ear 'you're not my friend and I don't like you,' etc. I was shocked at such behaviour from a 3 or 4 year old, and of course tried to keep them separated, but for some reason Sarah seemed to be completely under Molly's control and kept gravitating towards her, only for Molly to be awful to her!
My mum has told me that a girl who was horrible to me when we were about 9, was already showing 'bitchiness' aged 3 at playgroup. Apparently, when one girl came into playgroup proudly wearing some colourful homemade dungarees my mum witnessed her turn to another couple of little girls and say 'Eurgh, don't you think 's clothes are horrible?!' And then she openly laughed at this poor child. Aged 3!
Last year I taught Reception, and had one little boy who seemed to be a model pupil, but after a few days I sussed that the problems another couple of boys were having ('Lewis says he's not my friend!' etc) were actually being caused by this boy going up to one of the other 2 boys and telling him that the other boy had been saying things about him. Once, when trying to get to the bottom of a playground quarrel between a few boys I overheard him telling one boy what to say to me, which was untrue, and which would get another boy into trouble. He seemed to delight in seeing others upset or in trouble, and even complained to his mum about someone picking on him, so she came to the school to complain about bullying. He also made an allegation about a member of staff smacking him, which I was never told anything more about after telling a senior teacher, but as the staff member wasn't disciplined or sacked I assume it was a lie.
I think in most cases upbringing is the cause of most children's misbehaviour, but it is very difficult to get the balance right, as children with too many boundaries can grow up with problems just like children with too few boundaries!