Ethics, my friend, I would define as simply the golden rule.
Would you want it done to you?
Dumping 100,000 coins to intentionally crash the market only to buy back cheap may be profitable. But it sure isn't ethical to all those that you affected by that action.
Just because it's not illegal sure as hell doesn't make it ethical. Just ask Goldman Sacks.
Would I want what done to me? Someone selling 100,000 Bitcoins is doing nothing to me. I choose to hold Bitcoins and because of that choice I must accept the consequences of any exchange rate fluctuations. Anyone who is "affected" by someone selling Bitcoins has put themselves in that position by holding Bitcoins. The seller is not to blame for any losses the holder incurs. It's such a ridiculous proposition that it would render all trade unethical (it's simply a matter of scale)! As long as I am not forced to use Bitcoin (through legal tender laws, etc.) the question of ethics does not come up.
Banks like Goldman Sacks work inside a system built with rules they've chosen themselves. This is the type of market I would call unethical. This is why I say there should be no rules about who I can sell to (buy from), when I can sell (buy), or how much I can sell (buy). The rules which apply to some, but not all, are what allows these unethical practices to occur. When everyone plays by the same rules, everyone encounters the same risk.
Anyway, I'm not going to bother trying to convince you further.
Lots of new investors are entering this market now. And are easy prey for a hardened trader with resources to move the market. That sure as hell doesn't make it right to prey on them when the situations develop.
Your intentions and your ethics in your trades is your business. At the end of the day you have to look yourself in the mirror.
You, I, everyone, creates all kinds of confabulations to justify our unethical behavior. And the danger is self-delusion covers its own tracks. We don't even realize we are doing it.
...they key then, is to recognize when we are confabulating to ourselves. And oh, how few have that kind of self-awareness.
Careful the road you're going down, defining something as an "inside system" or whatever creative confabulation to make what Goldman Sacks did wrong, and manipulating the Bitcoin market right.
Confabulations only blind ourselves to the underlying truth.
At the end of the day, if you're preying on someones ignorance or simple naivety you're being unethical regardless of the context.
...And that always carries a consequence. No one is an island.