Only creatures on the battlefield when Death Frenzy resolves will get -2/-2. However, if a creature enters the battlefield and then dies later that turn, you’ll gain 1 life for it.

You gain 1 life for each creature that Death Frenzy causes to have a toughness of 0 or less. The delayed triggered ability is created before state-based actions cause any creatures with 0 or less toughness to die.