I have been wondering if Karma is activated by emotions similar to sharingan. Think about this... Boruto had to kill someone to obtain it. It first activated when he was about to finish off Shojoji. It activated again to save Konohamaru. All these instances were during the heat of battle when emotions run high.
When Kurama was the kyubbi he would take advantage of Naruto's negative emotions. Sasuke's sharingan fed off of his darkness. So it would not be far off track for Boruto to have an ability that feeds off negativity.
When Kurama was the kyubbi he would take advantage of Naruto's negative emotions. Sasuke's sharingan fed off of his darkness. So it would not be far off track for Boruto to have an ability that feeds off negativity.