Is it possible to do damage to a non undead with healing? Like too much of something should be harmful right? Does healing work like accelerating the rate of natural regeneration, then you could keep accelerating a healthy part of an enemey's body and make it grow older/weaker/even rot? Or even their whole body...
I am not aware of any mechanic that would allow you to do that with healing spells (my knowledge is limited though for editions other than 3.x or 5).
However, 3.x had the plane of positive energy, which pumped creatures there full of life, in the form of temporary hp. While initially beneficial, once a creature had accrued more than its initial maximum hp in temporary hp, it had to repeatedly pass Saving Throws to avoid bursting due to an excess of positive energy, killing it.
In a way, you could weaponize this trait by plane shifting a foe to that plane, where they would be “killed by healing“, unless they had some way to return or protect themselves.