Why is it important if they know it’s illigal or not? Seems to me the important question is what harm they are doing to others.

Possibly You are right, it is very hard for me to comprehend what goes on inside the mind of an addict. I would say for arguments sake that the addict is also aware of the damage that they are doing to others but unfortunately the desire for the drug overpowers any sense of morality. Short of locking them up which then puts the burden and guilt on the rest of us, or making all drugs legal and free, I am not sure what would work. Like I said my thoughts are quite conflicted with this matter.

My personal solution although fairly selfish is to avoid getting into and breaking away from any relationship if and when it becomes apperant that someone is using.

I think the least bad option is to let allow people to make mistakes and learn from them. How would you feel about sending people to prison for eating junk food, smoking a cigarette, drinking a beer, wasting time on social media/watching netflix? And afaik countries that have decriminalized drugs and focused on harm reduction actually end up with less drug addicts. Its also worth noting that most cases of overdoses are caused by the drug either being cut with something stronger or the user just taking too much due to high inconsitinsies in purity with each purchase

How would one ensure that it wouldnt happen again?

imo the risk of convicted child molesters having a hard time finding jobs involving children despite being deemed rehabilitated by some beurocratic organization is preferrable to the risk of giving them ample oppertunity to molest more children, unless it could be scientifically demonstrated with great certainty that the rehabilitation program was completely effective in like at least 99.99% of cases (how would you even falsify this hypothesis given how many victims choose not to speak out?). Similarly i would not want people such as bankers or politicians convited of corruption/fraud or similar things being in positions where they have significant influence over the economic conditions of millions of people without proper supervision (which clearly is there is not) The strategies for dealing with criminals should really vary depending on the type of crime thats been commited i think, as others have pointed out i think your views on this are too black and white

tbf doing things like GR or QFT is not even required for a graduate degree in physics depending on your field of research