Yes, killing will cause several bad consequences. For one, they don’t live long. Second, the mind is not peaceful. Three are panic and fear at the end of life. Fourth, when Yang Qi is weak, it is easy to be plagued by external diseases. Five of them will be reincarnated as dogs and killed in the next life.

There are many ancient book records and explanations in this regard. For example, "Buddha's Sutra on the Difference of Karma and Retribution" and so on.

Please do not underestimate the wisdom of the ancients. The summary of thousands of years must have its reasons and truths.