This would be the worst thing they can do right now. They don't stand any chance of winning against Russia if the Russians suddenly have a reason to hate them.
By playing only defense, they are a victim, even a martyr. Who doesn't empathize with someone having to pick up a weapon and fight for their country/survival. It's the stuff of songs and legends.
What might be nice instead is of Ukraine can buy a few French Exocet missiles and attack the Russian fleet in the Black sea. That might hit them where it hurts.
As I understand it, they have been attacking Russian ships. I guess there is some speculation that Russians have been using civilian shipping as cover, resulting in two non-Russian ships being damaged by fire meant for warships.
By playing only defense, they are a victim, even a martyr. Who doesn't empathize with someone having to pick up a weapon and fight for their country/survival. It's the stuff of songs and legends.