I know that the whole "oh, that was legendary magic? In my past life, everyone could do this, you know?" thing is a common reincarnation trope, but saying that about instant death magic of all things seems really dumb, lmao.
Like if it was a case of "oh, this legendary fire spell is actually basic magic we needed to use to defeat normal monsters", it'd make more sense, but if even he acknowledges it as instant death magic, that society seems pretty irresponsible. Just teach everyone instant death magic, what could go wrong?