My guess is that everybody regrets at least one event in history. Some of us regret entire periods like, say, The Dark Ages. But if you suddenly had access to the Wayback Machine and could go back and prevent some specific event, what would it be?
Some that have come to mind for me:
September 11, 2001
November 22, 1963
December 7, 1941
The firing on Fort Sumter
The murder of Bobby Kennedy
September 30, 1955
I’d like your input before I try to settle on the most dreadful event in history.
I think right now many will say 9/11, because we’re still awash in the aftermath. It was more horrific than Pearl Harbor, probably.
December 7, 1941
The firing on Fort Sumter
I think something along these lines would’ve occurred anyway.
November 22, 1963
To be quite honest, I’m not too sure that a two term Kennedy presidency would’ve made a huge difference. Less so then an RFK presidency, IMHO. I’m not that sure we really would’ve ended involvement in Vietnam with a continued JFK presidency.
The murder of Bobby Kennedy
This would be interesting. What would the liberal vs. conservative argument be like today if there had been a two-term RFK presidency? Nixon may have been too old to run in 1976 and Ford probably woudn’t have the GOP nominee. Interesting, but not the big one.
September 30, 1955
Not on the scale of the others. Maybe he would become Marlon Brando, who knows?
April 20th, 1889.
I’d be waiting to strangle the bastard as soon as his neck cleared Klara’s birth canal.
Either that or toss him into the oven!
Definitely one to add to the list!
How about?
January 30, 1933
Hindenburg names Hitler German vice-chancelor in exchange for Nazi Party support.
Dunno, I figure the good and bad of history is in what you decide to do about what you learned from it. Of course that’s a lot different from when it was the present.
April 15, 1865
Prevent Lincoln from being assassinated, have a slightly less punishment-minded Reconstruction but without the Southern apologism of Andrew Johnson and perhaps even prevent Jim Crow.
OR
mid-1947
Avoid killing my grandfather and sleeping with my grandmother.
Back to 1986 and buy lots and lots of Microsoft IPO stock shares.
It might not make the world a better place but it would prevent the huge injustice of me having to work for a living.
65 million years BC
Just to see what would happen.
That whole Cambrian extinction thing. Far more interesting than those silly dinosaurs!
Plus, Phnord Prephect beat me to it.
Rasputin endearing (if that’s the correct term) himself with the Romanovs.
If the revolution still occurs, I’ll have to think of something else.
If I could, I’d stop the destuction of the Library at Alexandria.
Send Kaiser Wilhelm straight to the Blazing Pit, long before he took the throne.
Another 50 years of European stability, post-Napoleon, would have been darn fine.
Or, kill Hong Xiuquan.
I am loathe to admit it here, but I think I might go about 2005 years back and snuff that pesky Jesus fellow.
Or better yet Abraham. Isn’t he the one considered to be the founder of Christianity, Judaism and Islam? Yeah, I’d go after that guy.
Didn’t they try that back then and it didn’t work?
Touche’ 
Ooh.
Good one.
Possibly preventing 2 world wars. This is genius. I’d give Bosda the prize. If you read into Wilhelm he was not evil like Hitler, just complete Hawkish Buffoon. Without WWI, conditions would not have led to Hitler’s rise.
Historically: December 7, 1941 was beneficial to the world. Japan kind of failed. No carriers and pulled USA into war overnight. We were still resistant to getting entangled in another European war until “The Day that will live in Infamy”
9/11 comes to mind, but since that’s taken…
The election, such as it was, of G.W. Bush in 2000.
Although I’m no particular fan of Al Gore, I think his adminstration’s response to 9/11 would have been much different. As saddening as 9/11 was in itself, I’ve found our response to it even more disheartening.