The 10th Amendment grants all police powers to the states unless enumerated otherwise in the Constitution (such as is the case with interstate commerce). And states may not enact laws that supercede federal laws per the Supremacy Clause. This is important when applied to arms as they are protected by the 2A in the Constitution, which is the highest law in the land.
As for general welfare, that’s targeted at federal spending. A common place where it applies is that it empowers the federal government to grant crop subsidies to farmers.
That’s why when someone commits a crime like murder it’s under state jurisdiction. Unless that particular case occurred on federal land or involved cross state travel, then that would become a federal case.