Mage Hack?

I wanted to know if anyone knows how to do this, Im sorry if this has already been posted but I searched and couldn't find anything.
So yesterday a random Water Mage joined my server and started killing others including NPCs. He was using normal spells but instead of giving health it was actually taking it away. Anyone know how to do this as I would love to know, I tried looking on CE but found nothing that managed to do this. Thanks

Trying my best to create a negative value, but it always ends up 'healing' for -2.147 billion - which doesn't kill me or my pet, but sets my health to -2.147b/-2.147b rather than just killing me.

I think the value has to be closer to 0, say -20,000 - but I can't create one like this just yet. Any help?