SPEAKER_00 says OpenAI Five penalizes killing an enemy hero because the kill yields gold that is already rewarded separately.

“What can always run into trouble with reinforced reinforcement learning is above there is a section where they have like killing an enemy hero actually gives a negative reward and the reason is and they have an explanation next to it is that killing an enemy hero gives you a lot of gold and there's already like a bonus for getting gold so if you also had like a zero positive reward for killing enemy hero you would sit into the middle of the map and just kill the enemy hero over and over and over again without making any progress.”