The first time some primitive human noticed that one plant produced better fruit than another plant of the same species, and cultivated the superior plant's seeds, he was "playing God." It's something human beings have done for millenia.
There is a huge gulf between humans and plants. It's about the soul, someone made in God's image. Thou shalt not kill means that it is The LORD who has the rightful decision as to what innocent human life is to live or die. If someone dies because of an accident, sickness or old age that is not immoral. If someones life is snuffed out by another person that is the sin of murder because although life and death does happen to humans it is God's territory and when we trespass upon it we are guilty before Him. We are not God and have not the right over the life and death of other innocent human beings.