Bit of both.
Interesting documentary I saw years ago about genes that make you more likely to be a psychopath. The researcher discovered he had them
but when he discussed it with his family, colleagues, other researchers they agreed (a) he had some of the personality traits you'd expect to see in a psychopathy but (b) he wasn't violent, amoral etc. He came from a close, loving family and his parents had worked hard on bringing him up with rules, boundaries, sanctions and so on, so he'd turned out OK.
Another interesting one from last year - lots of people who end up in prison for crimes of violence turn out to have had a head injury early in life.
Also, I think there is research that shows that experiencing neglect and/or abuse very early in life does actually change the way your brain develops.
And finally, susceptibility to addiction is genetic, and so are various neurodevelopmental disorders like autism spectrum disorder, ADHD, that can lead to problems getting on with others and functioning in a world designed for people with different brains.
It's easy to say that if an adult does bad things it's by choice. However, if you take all the above into account, making the right choices is a lot easier for some of us than it is for others.