The fact that Paarthurnax helped save the Kelpa means he's saved many times more lives then he's taken.
Hell, My Dragonborn has killed quiet a few people....
So in that case ít's a question of simple math?
Let's look at a few hypothetical situations then.
There is a train running towards a large group of people, for some reason I can't warn them. I can however throw one random guy in front of the train right now, forcing it to stop before it hits the other far larger group, should I be allowed too?
I've killed a 100+ people and intend to kill more, mostly for fun. But I also found a cure for cancer, I get arrested and tell the people that I will fabricate and distrubute this cure if I'm allowed to continue my murders. Would you let me continue or would you throw me in jail?
I'm a former warlord in Africa dealing in diamonds and stuff like that, my armies have killed and [censored] entire villages under my order, at age 60 after 40 years of doing this I felt sorry and started using my gathered wealth to build orphanages and what not, my programs have since then saved many more then I killed, I've become a public figure of hope as well uniting many people who now build for a better Africa. Nobody knows about my past. Now however you learn about my past, and want me thrown in jail, doing this will end all the charitable work I have done, would you go public and prosecute me? Or let me continue doing my good deeds?
In all cases math would suggest letting the (former) criminal go on, the end justifies the means and all that, but personally I don't think the end justifies the means is a good way to build a society. Sometimes strickt justice is needed.