He went out knowing he wanted to murder a woman , he took a bag of various knives with him along with masks to allow him to do it . Therefore he seemed pretty sane in his thinking , knowing what he wanted to do .
The ability to behave rationally to carry out an irrational thought does not necessarily constitute sanity, e.g. if the voices in my head were telling me to burn down a block of flats, and I did so in a methodical and precise fashion, is that sanity?