Saw this in one of my local comic shop's quarter boxes and I had to have it.
From
Supergirl Vol. 4, the regular series that ran from 1996 to 2003, written by Peter David with art by Gary Frank.
Issue three of the series has Gorilla Grodd acquiring a nifty pink gem called the Heart of Darkness and using it to bring out the bestial nature in the local townsfolk. It only works during a total eclipse, but luckily DC editorial snuffed out the sun for their big Final Night event, giving Grodd plenty of time to wreak havoc.
An early victim of the gem is Supergirl's friend, Doc Harcourt, who is overcome by concern for her brother's safety. It seems the gem feeds on base emotions, chief among them hate, using those impulses to warp people into monsters.
Supergirl rushes off to stop Grodd, but that doesn't go quite to plan.
On to issue four, which has plenty of post-TF action. Subverted to Grodd's will, Supergirl goes about doing evil and stuff.
That's her mom, and no, she doesn't kill her. It's a fairly dark story, though; several people are injured and at least one dies.
An ape after my own heart... Alas, there are no more changes in store for Supes, and she soon reverts to normal.
Then the sun comes out, everyone else reverts, the end. Grodd escapes and the gem isn't shown to have been recovered, leaving the door open for Grodd to indulge his apewoman fetish at a later time. Maybe he can find a way around that bothersome eclipse restriction

.