Why would it be wrong to regard someone dedicating most of their time to actively accomplishing ("working on") an unethical thing as an unethical person? What else could "unethical person" possibly mean?
I haven't censored anyone or shut down any argument by saying this, and neither was I attempting to, so your point seems to be confused.
You haven't shut down any argument by saying "anyone who works on this is a bad person"? Seriously? I happen to think the exact opposite: life extension is probably one of the most noble things we could possibly be working on. Imagine if Einstein were still alive. We never learn from history, maybe because the people who lived it are all dead now. But in your mind, thinking this way probably makes me unethical (i.e. bad)! It's no problem when we're both two blobs of text on a forum, but I certainly wouldn't want you to be a boss or someone I was forced to interact with on a regular basis. And if enough people behave as radical as you are they will likely want to go beyond simply labelling people (e.g. see all the "hate crime" talk).
I don't see how I can stop extrapolating from "action X is unethical because the expected consequences look like bad ones" to "this person who dearly wants to bring about action X is behaving unethically". I simply don't see them as being meaningfully different statements.
In your second paragraph you've deviated from what I'm talking about (perhaps I was responding to the wrong person? EDIT: I didn't). "Person X is behaving unethically" is perfectly fine. "This person is an unethical person because they engage in action X" is where I draw the line (asuming action X is controversial and not very clearly evil like first degree murder) because it assigns a trait to someone by which they will be judged by their peers.
EDIT: Actually no:
>people dedicating their lives to it today are unethical people.
You said that. So your revised statement is moving the goal posts.
> I haven't shut down any argument by saying this
You're being naive if you think that saying this doesn't end any hope of a reasonable discussion.
One person's reasonable discussion is another person's interminable sea lion. http://wondermark.com/1k62/
Given your stance here, you're perfectly ok with the baker refusing to sell a wedding cake to a gay couple, right? Because the law was effectively the sea lion in your comic.
Everyone is going to have some threshold where they think 'an act that is this immoral means the person doing the action is also immoral', where actions that are below that threshold are treated as 'this action is unethical, but the person doing it could still be good'
Most people think someone who murders is a bad person (the act is immoral, and the person doing it is immoral). However, as the we move down the line, EVERYONE has to 'make the call' as to where the action becomes something that doesn't immediately make the person doing the action immoral. It isn't wrong that we have to make that call, it is the only way.
If people aren't allowed to state that 'behavior X makes person Y unethical', then how are we supposed to determine what is 'objectively unethical'.