Firstly: It’s a fundamental mistake to try to judge the history of the 17 to 19 century with 21 century morality.
You should remember that there was an imperial competition. The alternative to being part of the British Empire was not independence; it was being part of the French / Portuguese / Dutch / Belgian or German Empires. Does anyone believe they would have been better off under those Empires? Look how the French left Indo-China, or the Portuguese left Angola. Look how many countries voluntarily continued their association with the British via the Commonwealth even after their independence.
You ask “how many were slaughtered?” You might also consider how many were saved. I have just lived in Sri Lanka for 12 years, where a civil war has claimed maybe 80,000 people since 1983. These are 80,000 who might be alive were the British still there. Does anyone think Zimbabwe is better off now ?
The contribution made in terms of railways and infrastructure could be approximately estimated. The economies the British restructured would be more difficult to estimate (They brought tea to Sri Lanka, rubber plantations to Malaysia etc). The contribution the British made in terms of the language, the rule of law, and the organization they left behind is beyond value.