If a person is influenced by his culture/environment, how can he be responsible for his choices? As our choices are determined by the past actions that affected us in some way.
This is a pretty broad question. I'm just going to do a brief overview for that reason.
Say you're at Diagon Alley and your tummy is making the rumbles that only Bertie Bott's Every Flavored Beans can satisfy. Since you're a wizard (in this example) you can throw on your invisibility cloak and take some without paying, or you can plunk down some hard earned cashmoney.
Well, chances are however you make your cashmoney, you'd rather do less of that, and eat more beans, so all things considered, you'd certainly prefer not to pay. However, you may choose to pay anyway.
What was that?
Oh, so there is a choice.
Granted, every choice is informed by past action. There's no denying that. However, whenever evaluating input to determine output, we must always make a choice. And this choices affect what additional inputs we receive and so on.
Well, at some point, we hadn't really received much input yet. Then the worst thing happened and we were born, and we immediately gained agency and were able to affect our surroundings, even if only in the slightest from the beginning.
So to me, our responsibility for our actions is seated with not just an individual actions, but the path through life we have chosen for ourselves and who we have decided, either intentional or unintentionally, to be.
So in that extent, we are fully responsible for our actions, as we have more control over them than anyone and even, to some extent, chose our culture and environment. Certainly, other people can influence us, and change our mental state, but in the end everything is our own internal choice.
Wait a minute. Turing Hypothesis. Everything computable in nature is computable by a Turing Machine, which means you're computable by a Turing Machine, and Turing Machines are deterministic. That means all your choices could reasonably be determined at any time given sufficient information about the present state and computing power (in practice, the uncertainty principle prevents this but can have a divine agent build the machine for purposes of philosophy). So while you are making choices, they're really determined by all the bits and bytes or tapes and symbols or dendrites and axons and you're just along for the ride.
Well, you get to decide whether determinism and free will are compatible or not as there doesn't seem to be any great degree of consensus on that issue. I hold that all individuals are completely responsible for their actions because they are the only agents that pass the final decision on their own actions so all responsibility rests with them. To me, it doesn't matter whether this decision is deterministic or not.
As mentioned elsewhere, this is a pretty broad (set) of questions so I'll give a pretty broad answer.
From a certain perspective, there are mitigating circumstances for any immoral action someone takes. We are all products of our upbringing, circumstances, and genetics to at least some degree. It may be true that a change in these factors can be the difference between a psychiatrist and a serial killer, but we (as a society) have a history of holding people accountable for their actions with very little attention paid to these factors. In short:
“We are products of our past, but we don't have to be prisoners of it.” ― Rick Warren