ProtoAvatar
Fleet Captain
Is it right to commit genocide to stop someone else committing genocide against you? Because that is what this argument boils down to, regardless of the weapons used (or not used).
Using the thalaron weapon against the borg is not genocide:
The borg in tha alpha/beta quadrants - all of them - are enemy soldiers on a genocidal mission against your civilization.
Using thalaron or any other weapons against them is pure self defence - allowed by both morals and the rules of war.