A loan to buy a cryptocurrency is not the best solution. I would even say that this is a crime against risk management. However, this can be done by following certain safety rules.
1. Borrow only the amount that you can pay off without any problems, even if you lose all your bitcoins.
2. As soon as after the purchase the value doubles, sell half of your bitcoins and repay the loan.
You are contradicting yourself with the 2 statements you made.
You are saying that one should not take risk and even termed it as crime, so you believe that chances are user won't be able to even earn enough to pay the interest.
But on the other hand, you are expecting the value to double up and pay off the loan and keep the remaining 50% which is no doubt a dream scenario but doubling happens very rarely.
I don't like the idea of taking loans and buying Bitcoins but if you know what you are doing and have the belief that you will not panic, then it might just work out.