How will prison help to make them less violent? And when they're released in 3 years, how will you ensure they don't hurt anyone anymore? We're not saying "don't do anything about criminals", but the current prison system is very explicitly about punishment (and, in the US, about slavery), not rehabilitation. What's needed, more often than not, are psychologists, monitoring and a way to keep them away from their victims. That does not require putting people in cages.