*I suspect if you analysed their backgrounds the common feature would be growing up in households which were not violent, hitting or intimidating people is not the answer and where they were taught empathy.
So how do we break the cycle of violence?*
It's huge and multi layered. Robert Webb's book how not to be a boy has a lot in it that illustrates the pressures boys are under - as you say though, not all men are violent and he certainly had a very empathetic side from being very young.
Every part of society is involved in this, so all parts of society need to change. Gender roles are still very ingrained subconsciously everywhere which is a factor. There's been a lot on empowering women / girls over the years but as the parent of a boy I feel boys aren't being focussed on in terms of the flip side to 'girl empowerment' - that boys can be extremely kind and empathetic - in books and the media and films. It's right to work tirelessly on girls' stereotyping, of course it is, bit I'm not seeing the opposite being done consistently for boys, personally.
I fully believe in nature then nurture, or the two together. I do wonder if the fact that some children are born more or less naturally empathetic (R Webb describes helping an injured bee while knowing he was 'supposed' to stamp on it) - not gender specific - but also that how a child is nurtured can impact that empathy (children taught about theory of mind became better at lying for example, they could also become better at being empathetic to others' feelings). However as girls generally get a richer exposure to empathetic caring upbringing via wider society outside the home (eg toys, books, stories, tv) and boys less so, and may be actively encouraged to "toughen up" at home, things become unbalanced.
We've even had nappy ads recently still perpetuating gender stereotypes. There's a constant drip drip drip of subtle inequality in how children should be thought of by adults according to their sex.
Until I had a boy and until I ventured into the feminist boards of mn I was oblivious to it. And yet thought of myself as progressive, feminist etc. I went along with the whole "it's typical boys behaviour." A child in my son's class who we've known since birth is very immature and boisterous and pushes and hits other children and being tall and strong dominates them easily. I remember his mother saying that Nursery teachers reassured her over this behaviour, saying "it's boys! That's what they're like!" Ie don't worry too much. I suspect there'd be more concern about psych referrals if it was a girl doing the same.
(Oops, longer than I meant it to be!)