Interesting. I had an Eclipse and Antimattered Deathstalker out, and when the Eclipse was destroyed, the Deathstalker's health was dropped to -4.
Wait, the health? Or the attack?
If the attack, I think this may be intended.
Card played (Atk)
Play Pest. (0)
Eclipse. (2)
Antimatter. (-2)
Destroy Eclipse. (-4)
And if you want to keep going...
Antimatter. (4)
Eclipse. (6)
Antimatter (-6)
Destroy Eclipse. (-8)
Antimatter (8)
Eclipse. (10)
Antimatter (-10)
Destroy Eclipse (-12)
Antimatter (12)
Eclipse. (14)
Antimatter (-14)
Destroy Eclipse. (-16)
Antimatter (16)
Eclipse. (18)
Antimatter (-18)
Destroy Eclipse (-20)
Antimatter (20)
Eclipse (22)
Antimatter (-22)
Destroy Eclipse (-24)
Antimatter (24)
*Repeat above steps once*
Final Antimatter (48!)
Because Eclipse only acts upon the current attack.