I don't think this is quite true, it may have ended it faster, but I don't think it would still exist today if the civil war had not happened. Most other countries ended slavery without a violent civil war, especially if you think about the way technology vastly outweighed the usefulness of having slaves.
And then when Charlie Kirk says "Some deaths were worth it...", he is talking about accidents and abuses of guns by shooters. He doesn't mean that violence is the answer to politics, it would be great if nobody died from mass shootings. But he is saying that having the right to bear arms to defend yourself is preferable to the alternative where you have no right to do that.