Why was George Washington put to death in October of 2007 ?

I'm pretty sure he was severely injured on the gallops at Newmarket, rather than in a race.

Yes, he was injured, though it was during a race, The Breeder's Cup to be specific. Thanks for the answer !

George Washington KIlled (Source)

Yes. I saw him win the 2000 Guineas at Newmarket (the first of the Classics for which colts are eligible) and finish second in the Derby at Epsom. He had a fine turn of foot, which saw him home over the mile of the Guineas but was only just able to get a mile and a half, so the Derby , at that distance and over the difficult switchback of the Epsom course, was just a bit much for him. He'd have needed to 'hear his feet rattle', have fast firm going, to get very close to winning there, though he did make second.I also saw him win the Queen Elizabeth Stakes at Royal Ascot (one mile ), again at three years.As a measure of the doubts over his stamina he was a good price with the betting public when quite a lot of rain fell and supposedly softened the ground. It's a measure of the stupidity of the betting public that they don't appreciate that at Ascot a lot of rain falling the day before a race soon drains straight through the turf, so the going is not affected the next day!

He was kept in training beyond three principally because of doubts over his fertility.[The dread fashion now is to retire top colts to stud at three, if they win a Classic,which is a bit like retiring an athlete for winning as a teenager, since a colt is not a horse, nor a filly a mare, until five years] We were looking forward to seeing him mature into a strong entrant and winner in further big races when tragedy struck