This one is a contentious inclusion for two reasons. Firstly because it took place outside of core DC continuity, in the alternative universe event Injustice: Gods Among Us, and secondly because the Joker got somebody else to do his dirty work.
That somebody was Superman, and all it took to trick him into killing his beloved Lois Lane and triggering a chain of events which brought the DC Universe as we know it to an end was a whiff of the Scarecrow's fear gas. Although it was Supes who fatally wounded the Daily Planet's finest, this goes down as the Clown Prince's kill, as he orchestrated the entire thing by using the toxin on the Kryptonian.
It wasn't just Lois the Joker kills on this occasion, as the villain had also synced her heartbeat to a nuke, which detonated when she perished and wiped out Metropolis.
An enraged Supes' response was to ram his fist through Mister J's stomach to kill him, thus crossing the line between heroism and villainy. The Joker's catalytic role in Injustice ended up costing him his life, but it was all part of the plan.