Poison self damage, Tainted pact + golden rule + bladefall bug

My friend is playing Bog Shaman with tainted pact, golden rule, everything works fine but poisons from bladefall of trathus are self damaging him instead of healing.

Poison builds tend to focus on Damage over Time rather than damage from hits.

If the hits aren't doing enough damage to leech enough life that the leech has a duration instead of being instant, you won't see any Tainted Pact working at all. Even if you do, if the poison DoT is great enough, it wouldn't be surprising to see Tainted Pact heal the character very quickly, ending the leech because you reach full health--making you take damage from the poison again.

On top of what Jadian said, I want to say a lot of those builds typically go Slayer or some other gimmick to extend the leech via overleech even though HP is full. So they maintain the leech between packs.

Slayer also has the extra benefit of extending the overleech to 10 seconds.