Say Sarada wants the Mangekyou Sharingan, would this mean she would have to kill Boruto?
She wouldn't have to kill anyone herself, she just needs to go through a traumatic event like seeing someone close to her die e.g. Boruto, Sakura, Sasuke, ...
I'm not sure how Madara got his Mangekyou Sharingan, but he didn't kill his brother himself. Izuna gave his eyes to Madara on his deathbed to awaken the eternal Mangekyou Sharingan.
From the Wikia:
It is initially awakened by the trauma suffered from witnessing the death of someone close to the user. Uchiha has historically misunderstood these criteria as the user needing to be responsible for the person's death, and for that reason, they developed the practice of killing their closest friends in order to gain the Mangekyō Sharingan.
Not necessarily. To awaken the Sharingan, the person must go through a trauma or extreme emotional feelings.
If Sarada kills Boruto, she does have a strong chance of gaining the Mangekyo Sharingan considering their relationship.
Sarada can gain the Mangekyo Sharingan if she
- Kills a friend or a close family member or someone whom she trusts and loves.
- Witnesses the death of someone very dear to her.
- Goes through something more traumatic or emotional.
She could obtain the mangekyou by killing someone close to her, going through a tramatic event or having deep emotions and we know this when sarada and Chocho followed the 7th hokage,Naruto. They followed him because Chocho though choji was not her dad and for sarada uchiha she wanted to see sasuke, Sarada's dad and when hearing where sasuke is sarada runs off crying and activates the sharingan a sacred dojutsu of the uchiha clan.