In the middle of a game and the Warrior has acquired the Poison Dagger.

His card says he MAY roll two dice and choose the higher score.

Does this mean one can choose to just roll one die in case a 1 is rolled?

If two dice are rolled, can one choose the lower score or must the highscore be chosen?

I would say he can choose to roll one if he wants to, but must choose the higher result if he rolls two

I would say he can choose to roll one if he wants to, but must choose the higher result if he rolls two

It's correct.

The abilty is "You may roll two dice in battle and use the higher attack roll to determine your attack score."

NOT "You MAY roll two dice in battle and MAY use the higher attack roll to determine your attack score".

So, the Poisoned Dagger is not so useful to the Warrior, except when he rolls a double 1 or chooses not to use his ability.

Thematically, a Poisoned Dagger is a Thief's or Wizard's weapon, not for the bold Warrior who relies on his weapon mastery to win battles.