Do you believe there is a practical way of ensuring justice will be served? Is there a way of convincing Russia's leaders of turning themselves in for punishment? That would seem a bit naive. Remember, no laws between nations. Even if there were a way of ultimately achieving justice, of what value would it be if there are hundreds of thousands of avoidable deaths if a live let live philosophy was employed?
Putin may privately regret invading Ukraine at this point, but how to exit without overwhelming costs? The excessive burden placed on Germany in WWI lead to WWII, right? Negotiations will be tough, but how about the alternative? Ultimately, to beat a point to death, the participants will need to go to the peace table eventually, anyway.
As far as the idea of Putin remaining a future threat even if peace is achieved in the short term, it is a potent one deserving an answer. Earnest peace talks may give clarity on that point. Understand if the conclusion is Putin will remain a long term threat, the logical course of action would be to escalate in every way possible and commit to taking the initiative, including invasion of Russia and destruction of her military and economic targets. What do you think happens then? What happens to the support of Western political leaders by voters when the war hits their home countries?
It seems neither the hardest or softest options are optimal. An earnest effort as discussing the issues at the peace table will hopefully lay the foundation for la sustainable long term agreement.