(sorry for late reply, been away past couple of weeks)
Mainly for what he did in India. He (in his army capacity) was quite brutal in suppressing the early independence movement (destroyed people’s houses, burned crops in poor agricultural areas, etc.). He is quite loathed there even to this day, and understandably so.
He also bragged that he personally shot 'savages' in Sudan; advocated concentration camps in South Africa; called for a programme of imperial conquests because he believed that "the Aryan stock is bound to triumph" (if you heard that quote in isolation, I'm sure you might assume it came from another key figure of that time!); he supported/backed the Black and Tan thugs who terrorised Ireland's Catholic civilians; he also once said "I am strongly in favour of using poisoned gas against uncivilised tribes. It would spread a lively terror" when discussing taking action against the Kurds who rebelled against British rule.
There are also many quotes attributed to him that, in my personal view, make him come across as a bit of a tw@, but because he was old, British and had a funny face we just put it down to bulldog spirit and hijinks.
In summary, he was an aristocrat who viewed anyone who wasn't white British with imperialist contempt; which is ironic, given that he is known for his role in fighting against German white supremacist imperialism! But because he was on our side we seem to laud him and put him on our money.