  The interaction of a lattice of localized magnetic moments with a sea of
conduction electrons in Kondo lattice models induces rich quantum phases of
matter, such as Fermi liquids with heavily renormalized electronic
quasiparticles, quantum critical non-Fermi liquid metals and unconventional
superconductors, among others. The recent demonstration of moir\'e Kondo
lattices has opened the door to investigate the Kondo problem with continuously
tunable parameters. Although a heavy Fermi liquid phase has been identified in
moir\'e Kondo lattices, the magnetic phases and Kondo breakdown transitions
remain unexplored. Here we report a density-tuned Kondo destruction in
AB-stacked MoTe2/WSe2 moir\'e bilayers by combining magneto transport and
optical studies. As the itinerant carrier density decreases, the Kondo
temperature decreases. At a critical density, we observe a heavy Fermi liquid
to insulator transition, and a nearly concomitant emergence of ferromagnetic
order. The observation is consistent with the scenario of a ferromagnetic
Anderson insulator and suppression of the Kondo screening effect. Our results
pave the path for inducing other exotic quantum phase transitions in moir\'e
Kondo lattices.
