Unfortunately, Gaddafi was murdered by revolutionaries before he could be put on trial. He should have been tried in a criminal court before being sentenced to death. The former leader commited atrocious crimes, but he needed to be found guilty in court before further action was taken with his life.
In order to have a functioning civilized society, even the worst war criminals deserve a fair trial, even if the outcome is going to be the same. We've been seeing the United States support a lot of executions and kangaroo courts in other countries recently, and it's deeply upsetting to witness.
I believe Muammar Gaddafi should have been tried for his crimes as the Libyan leader, rather than killed by revolutionaries. I believe it is better to have these figure heads stand trial because more information can be gleaned for the history books, however there is no way to change the past.
In an ideal world Muammar Gaddafi would have lived to face his crimes in trial and then appropriately been punished. I, however, cannot feel bad for the man and I am not offended that the revolutionaries took out all of their hatred on the man who caused it all to them.
No, Muammar Gaddafi should not have been tried for his crimes as Libyan leader, rather than killed by revolutionaries, because the revolutionaries were able to exact quick justice. If he were tried, the process likely would have taken years. He would have led discussions about his rights, lawyers, and whether Geneva protections and the constitution apply to him. Revolutionaries got the job done quickly and better.