Should I be honest with postdoc candidate about awful lab working environment?
The general rule in these situations is that being honest is okay (though consider that it can be risky if word of your honesty makes it back to the boss), but it's important to stick to facts and to your opinions about your own situation/experience.
"The team is destructive" is not something you should say, nor is "the lab is mismanaged". Those are your opinions about other people: stay away. What you can say is "I feel like I've had trouble getting credit for my work", "I wish I had more freedom to choose the direction of my work", etc. Even better is to use specific factual information (note: opinions are not facts), like "I've only been able to get one first-author publication in the past three years."
That said, take care of yourself, too. If you need a recommendation for a future job, it might be necessary to bite your tongue a bit. It's good to want to warn a candidate off gently, but consider that once you are gone someone is going to get that position, and it's even possible that the person who does will have a different experience than you did.
Also related: https://workplace.stackexchange.com/questions/137037/can-i-tell-a-prospective-employee-that-everyone-in-the-team-is-leaving (not an identical situation but I think the advice there still applies).
Yes, you should be honest, but you may be able to do it without jeopardizing yourself or assigning blame to individuals. You can also answer "no comment" to questions that you think would leave you vulnerable, and the candidate is likely to get the proper implication. In fact, stating that "Off the record, I'd prefer to be elsewhere" is the poison pill that the candidate will probably find sufficient.
If not asked, it is a little harder to make a recommendation. Don't leave yourself open to retaliation.
In this situation, I believe in being a tactful truth-teller but not a complainer.
You are entitled to describe your opinions and feelings (truth) but not those of others (gossip).
Example
Q: What's it like working here?
A: I have to admit the approach here feels a little too combative for me - I'm sure everyone has their own opinion.
If you are worried about your words getting back to the wrong people, remember that they can't quote what isn't explicitly said. I believe that a lot can be conveyed with a pause, a sigh, a wry expression or a non-committal answer.
Examples
Q: What's your supervisor like to work for?
A: Next question! (blunt non-committal - preferably not in the hearing of the head!)
Q: What's your supervisor like to work for?
A: Mmmm ... We have our moments. (non-committal but more gentle)
Q: What's your supervisor like to work for?
A: What sort supervisor are you hoping for? (evasive - letting the other person specify)
Q: Oh someone who is easy-going, relaxed and generous.
A: Smiles and says "Might not be your type then"
I could give hundreds of examples but what's really important is your mind-set. Vow not to lie. Be calm and act as though you are describing what happens in a movie, i.e depersonalise but answer truthfully (or possibly decline to answer).