Is it irrational to take out yourself as well as your enemies if you have no realistic prospects for improving your situation but you do have the ability to hurt people who hurt you?
Yes, I'd agree it's irrational to willfully give yourself a chemical addiction in a place where you are already have enough stressors and it'll be hard for you to continue to feed that addiction once started. It's probably a terrible choice to start an addition in a concentration camp. I can understand why people would choose it, and I don't fault them for making such a decision in such a bad place, but it's definitely not the rational choice.
Just purely from a self-improvement standpoint, you should try and not assume you know everything about someone through a short conversation.
I'm not passing off a moral judgement over someone choosing to smoke in a concentration camp. I don't fault someone making that choice. There's a good chance I might make the same. That doesn't make it a rational choice.