“Can too much CO2 kill a plant?” Yes, but not at any level we are capable of generating. You clearly are not too familiar with basic biology, in spite of the caps: Market Gardeners keep their tunnel-houses artificially pumped with 1000 ppm CO2; that is the optimum for plant growth. We are talking about CO2 levels of only 400 ppm for the planet, well below optimum for plant growth but far better than the starvation level of 240pp in 1800. With the best will in the world, after 200 years of burning fossil fuels as fast as we can, we can’t get it much higher than that. There is zero chance of any plant life on Planet Earth dying from too much CO2. Zero.