I have only played this card in ONE deck. and that deck was shotgun Harrigan where it performed pretty well actually.
I was all in on shotgun, which late in the campaign included a custom modification, custom ammunition and an enchant weapon.
So I was frequently dolling out 5-8 damage on basically any attack, making relentless actually kinda good as every enemy slain was being massively overkilled, I once discarded relentless for about 20-something resources.....
In most other decks this card is bad. Most decks are not built around over-killing enemies becuase well.... Overkilling usually means you spent more effort than it was worth to kill something, in which case getting compensated 1 resource per damage is not a great rate.
Would love it if they made an XP version of this card though.