Two-Face wants Erin McKillen dead, that's for sure, but first, he wants Erin to apologize to the grave of his wife, Gilda... something Erin sarcastically does, which doesn't please Harvey. The coin is flipped, heads, Erin burns, tails she dies... aaaaand HEADS! Harvey is all set up with some acid, ready to inflict the same pain on Erin, as she did to him, but then the rest of the McKillen family show up, firing a shot through Harvey and the acid (some of which gets on Erin's face).
The McKillen family is trying to kill
And it does, so Harvey reluctantly accompanies Bruce and Erin into some old bootlegger tunnels, but it doesn't take long for the mob to give chase. With the use of an R.P.G., the mobsters cause a collapse that separates Harvey from Bruce and Erin, but doesn't kill him... they'll save that for later.
Time for a flashback! Turns out Harvey was a defense attorney before he became the D.A. getting criminals off on technicalities, because Batman doesn't read them their rights. Turns out he was also the McKillens' family attorney, and when he learns about a hit on Gordon they placed, it doesn't sit well with him, but his hands are tied due to attorney/client privilege. This is the seed of doubt that gets planted in Harvey's head, eventually allowing himself to get talked into running for D.A. when Bruce Wayne suggests he goes for the job. First oder of business when he becomes the D.A.? Take down the McKillens.
Bruce and Erin emerge into the bar the tunnels lead to, and soon find a broadcast going out to all TV's in Gotham, originating from the mob. They've got Harvey tied up, and are offering Gotham City residents their home back, by getting rid of the freaks... Harvey being the first to go.