I feel bad because the world is being made the shithole it has become, but that's just the way life is. I still see myself as more of a victim than a perpetrator: Millions of morons keep flooding the welfare system with offspring, and responsible people like myself have to pay for it. Do they feel bad? No - they just follow their instincts and never even think about the lives they are destroying.
Down in the village the inbred hicks keep opening brothels whose main advertising venue seems to be the noise terrorism they inflict upon us with their high power stereo systems. The same goes for deliberately modified motorcycles and boats racing back and forth and back and forth for no reason but to pollute the environment with toxic chemicals and noise. Do they care that they cause people like me to grow tumors and have heart attacks? I don't think they know that such conditions exist. They just follow their instincts.
Now I would want to hear one good reason why I should have a problem with providing liquidity to a bad trader? On the contrary, I make his loss a little smaller by offering him a slightly better fill than he would receive if I were not here (for example if he had run me over with his boat doing 200 mph in the zone where little children play).
If anything, I feel bad for improving his bottom line by those few cents, because that will enable him to reproduce and rampage a little longer before he finally goes broke and ends his vegetative existence.
Even if I contibuted a little part to driving him closer to death, I would still be doing him a favor. It's not like those individuals are happy doing what they do, they just follow their instincts. And their genes don't care for their happiness, the only thing they are built to maximize is their own reproductive success.
And like some others have mentioned, I don't think there are a lot of professions that are more ethical than daytrading. Think about it. If you try to make a living as a noise terrorist (construction worker, rap "musician", etc.) you force damage onto innocent bystanders. If you sell cigarettes you take advantage of the mental shortcomings of the average human and cause him a slow and painful death, plus you also damage innocent bystanders through second hand smoke. If you sell any form of physical merchandise (short of something like tomatoes from your back garden) it contributes directly to the destruction of our natural resources.
If you trade, then all you do is provide someone who has made an independent decision (to trade the other side) with a slightly better fill. Other than that, you don't change anything.