I think it depends what your concern is. Extreme example, but if their CV admits they used to work as a con-artist doing street hustles, then it would be legitimate for you to wonder whether they can be trusted with your clients even if they've done their time (assuming it's legal in their particular circumstances to consider their criminal record), or even if they weren't convicted due to lack of evidence.
If they worked as a lawyer for a firm that specialised in vexatious litigation, then you might be concerned they'll bring a gung-ho attitude to their new role that's out of character for your firm.
In each case, the thing to think/talk about is whether they will treat your clients honestly rather than trying to trick them, and whether they are excessively willing to bring unmerited legal actions. Avoid their ethical or political assessment of their former employment. Talk about their approach to the job they'll be doing, and how they've previously handled tasks similar to what will be required, and keep an eye out for them advocating shady practices. Don't make disavowing his previous line of work a condition for employment with you, just do enough to be confident that whatever it is about that line of work that conflicts with the requirements of this job, he knows not to bring it with him.
Also, be sure that the issue relates to the job they're applying for. You say that you consider it ethically questionable, but more importantly does the employer they're applying to, as a distinct entity from you the interviewer, consider it ethically questionable? If you just happen to think that butchers are unethical because you're a vegetarian, that's not relevant to a job selling double-glazing. Next, be sure that what the candidate did poses a potential problem before going into it: treating the typist at an arms dealer who sells landmines differently from the typist at a kitten sanctuary, or asking them to justify their willingness to work there, is probably just prejudice.
All you're really doing then is to make sure he's "a good fit" to carry out the goals of this company, and do the job the way you think it should be done. Which is legitimate.
In some cases there could even be an obvious "poacher turned gamekeeper" scenario, like if the MPAA hired someone who formerly worked for Pirate Bay or vice-versa. In that case it would be strange not to talk about how they can apply their past experience on the other side of the argument.
Aside from this, if you have a potential PR issue on your hands ("Former concentration camp guard employed by State Department!", "CEO of LovelyFamilyCorp sold cluster bombs to Russia!") then their history genuinely is relevant even if it was legal at the time.