If murder aside from abortion were legal in any states, I believe it would be appropriate for Federal action. The Feds have historically done this over many things that are not explicitly guaranteed by the Constituiton when they did not like the way States handled it.
Things like voting regulations such as poll taxes and ballot tests.
Things like public schools administering.
And even things like criminal trials that did not go their way hence Civil Rights violations.
And of course the big daddy that so many love to usurp for any argument here....slavery.
One, I don’t agree that federal intervention in all your examples was warranted or beneficial.
Two, even when federal intervention was warranted, it generally did not require a Constitutional amendment, and such intervention was also supported by a fairly wide consensus.
Three, I don’t buy into hypotheticals like states legalizing murder. I know that there are many who equate abortion at every stage of pregnancy with murder. I do not. Feel free to disagree, but remember that some who have other items on their agenda, as I do, actually want to win an election from time to time, especially if it can be won by somebody who shares one’s limited government philosophy.