You can say the same thing about National-Socialism or any other ideology out there. There are just not enough countries to try them all on, and I am not sure why should we...
If you want to make an argument that the principles of communism are unethical, that's an independent argument from the claim that certain historic attempts had various atrocities and therefore you reject the principles (even though many of the atrocities are not related to the principles — Stalin disappearing his political enemies has no basis in communist principles…
2. Specifically about giving communism a fair trial despite all past failures because it is "ethical": no thanks. I find its principle of "dictatorship of the proletariat" unethical and proven disastrous.
3. Stalin disappearing his political enemies was rooted at least in part in Lenin's cornerstone principle of "democratic centralism" which was critical ingredient for the success of the communist revolution. Trying to decouple the two doesn't do history justice.
There's definitely philosophical problems with communism and communist thinkers of various sorts. No denying that. But the screwed up crappy attempts at something like communism simply don't invalidate all of the principles which must be considered separately.
Basically, if you run a shitty version of an experiment, you don't get to then claim you proved the null hypothesis. It's one thing to say "we can't afford to experiment" and another to say "we had [shitty] experiments and so now we can conclude things."
You could argue for no more trials and still accept the fact that we never got any good trials from which to conclude enough about the real principles.
I'd also contend that it's unethical for a society to reward all people the same regardless of effort and talent.
Should I make as much money as a singer as Taylor Swift? I have terrible pitch. I can't write music. I can't play an instrument. I have no stage presence.
Now, you might say that based on my talents, I shouldn't be allowed to be a singer at all. But not allowing someone to choose what they do with their life also seems unethical.
That's not the premise of socialism, so it's strawman argument.