From initial reading, it appears that killers or killer is a variation of black ball, given that it uses the same table layout.
I could not find a definitive guide to killer rules, but there is a Wikipedia article. It contains this pertinent information:
Usually if the player scratches then an additional life is lost.
Scratching usually means pocketing the cue ball, but can also mean failing to contact a ball.
I would say that in any game that is serious enough to have players complain about cheating, this rule (ie. losing two points on a foul) would probably apply to prevent perverse incentives arising. Having said that, you were on your last life, so it wouldn't have made much difference here anyway.
If your opponent failed to consider that the player taking the shot before him had nothing to lose, that's his problem for not preparing accordingly. There's nothing to say that had you made an honest attempt, that the resulting leave would have been any better for him.
Deliberately fouling in pool games is generally not seen to be "cheating", as the penalties for fouling are generally strong enough to prevent the affected party from feeling aggrieved. This discussion seems to share that consensus - in 8-ball, for example, the ball-in-hand penalty is seen as a strong enough disincentive to deliberate fouling.