Would it honestly have made a difference if they had conducted a trial?
He was clearly going to be killed anyway. The brutality in the events leading up to his death are fairly worrying though. He was sexually assaulted with a pipe before being shot, this is no way for anyone to die. If this continues to be the way that justice is served then nothing will change for the Libyan people. Different factions of the rebels have already started bickering over who contributed more to the downfall of the regime.

Yes, it would have made a big difference.
One way is a murder, the other is justice. I don't know of any country where you can walk around killing people who 'will be killed anyway' and people think that is fine.

Besides, we don't know if he really would have been executed, since there was no trial.

Do you honestly believe that there is any doubt that he would have been executed? I didn't say it was right that he was killed either. I just think that given that it was pretty much a certainty that he would be put to death, it doesn't make a difference if he was murdered before or after the "trial". Neither of these scenarios are "justice".

Yup, murder is murder, I agree with that. But some countries say that executions are justice, for example a lot of middle eastern countries, and USA all have executions.

However the trial and verdict are the important parts. We cannot start enacting punishments before the trial on the basis of "well I definitely think they did it". There are multiple reasons for this not being a good idea;
1) Evidence must be presented - Gaddafi may have done some bad things, but he did also do some good things (free health care, free education, created low unemployment and poverty levels, created high literacy and standards of living, etc). Not everyone is going to condem him on the spot - ESPECIALLY having only seen what the mainstream media says (not saying that you know only what the media have said, I am sure that you checked independent sources etc, so don't take that personally).
2) The judgement must be decided by the trial - that is the main role of the trial.
3) Everyone has different views. You may say "there is no doubt", but others may not agree.

Simply walking around and punishing people without trials sets us back thousands of years...