She can be killed and her head taken at any time during the game. Due to its status as a quest item, it cannot be removed from the Pip-Boy inventory by any other means, aside from console commands.
Killing her with a headshot or by any means that damages her head reduces the price of her bounty to 50 caps and it becomes Violet's mangled head in the inventory.
With the Animal Friend perk her dogs will not be hostile.
With the Bloody Mess perk, it is possible that even when using V.A.T.S., her head will explode or be damaged, decreasing the reward.
If she is killed and her head taken prior to accepting the quest Three-Card Bounty, she can respawn, allowing a second head to be obtained. Should two or more heads be obtained, they cannot both be turned in for a bounty, extra heads can only be removed with console command (player.removeitem f81be X to remove intact heads, or player.removeitem 104173 X to remove mangled heads. Replace "X" with number of heads to remove).
