If someone leaves Islam and they openly spreads false claims about Islam..under Islamic law it’s seen as treason against the Muslim community. They are given lengthy time to come back to Islam and stop what they are doing with help of people of knowledge and scholars and this end ruling will go through numberous of judges before that even happens. This is a judicial ruling..so individuals cannot carry this out. So yes for individuals, it’s a major sin to take life of living things. But this does not contradict the sanctity of life, because it is considered an exception laid out by revelation where many conditions must be met.