Jamieson, Fausset and Brown on Jeremiah 19:6

6. no more ... Tophet--from Hebrew, toph, "drum"; for in sacrificing children to Molech drums were beaten to drown their cries. Thus the name indicated the joy of the people at the fancied propitiation of the god by this sacrifice; in antithesis to its joyless name subsequently.

valley of slaughter--It should be the scene of slaughter, no longer of children, but of men; not of "innocents" (Jr 19:4), but of those who richly deserved their fate. The city could not be assailed without first occupying the valley of Hinnom, in which was the only fountain: hence arose the violent battle there.