Representing criminals is fine, but aiding them in committing future crimes isn’t. If you do that, you’re just part of a criminal conspiracy, and being a lawyer doesn’t give you an exception from moral culpability.
I think the GP meant 'aiding them in commiting future crimes' in the literal sense (e.g. helping launder money, abusing attorney privilege etc.) rather than implying that by defending them in court the lawyer would then be culpable.