Apparantly, plants may in fact scream when they are stressed. Although this might sound ridiculous, considering how some plants can communicate with one another, informing other plants of their pain vocally could be possible.
So the way the researchers tested this, was by placing microphones next to plants, and then they cutting the stems of the plants (aka brutally murdering them). The plants emitted an ultrasonic signal which could be interpreted as a way to inform other plants of their unfortunate situation.
So the only logical conclusion would be, humans need to learn how to photosynthesize if they want to avoid slaughtering other living organisms.