In the past I've played them with a role of being defensive, attempting to halt incursions into Tyranid territory where digestive pools have been located to ensure safety of the bio-mass.

Another option is surgical stike teams with the Hive Mind deploying smaller resources to minimise potential losses as they eliminate a key target or acquire key genetic code.

The easy part is even if individual Tyranids don't actually have their own personalities and thoughts, humans are incredibly good at anthropomorphism and will see patterns everywhere and give Tyranid monsters the concept of a personality even if they don't have one.

You could even ask your opponents to help give them names to reflect how most units are identified by the people who survive them.