Gun control certainly won't solve everything, but that does not imply that it's not worth doing. You say so yourself - "Sure the number of mass killings with decrease because it won't be as easy". I agree, and that seems like a pretty decent thing to me. Sure gun control isn't a panacea, but anything that can help reduce the number of mass killings is probably a good thing.

Furthermore, I think a plausible argument can be made that restricting access to guns in general (especially handguns, assault rifles, and other non-hunting guns) will, over time, result in a cultural shift away from guns. We're talking a cultural change, so I'm aware that's a hard thing to predict, but I think it's certainly a possibility.