https://en.wikipedia.org/wiki/Slavery_Abolition_Act_1833
The UK abolished slavery, and even fought commonwealth territories that resisted. Maybe if parts of the US had still been in the commonwealth, slavery would have ended sooner there too.
If you look at the way Western Countries transitioned from one form of govt. to another its seems to have been very bloody. Glorious Revolution, French Revolution, American Revolution, German Revolutions, Russian Revolution etc.
So regicide doesn't count as violence? Also, as mentioned down-thread, we had a pretty bloody civil war here, which replaced the monarchy with a 'Lord Protector' which ultimately amounted to the same thing by another name and so we decided the monarchy wasn't that bad, as long as parliament kept the worst excesses in check.