連用形＋は＋せん　（殺しはせん。） meaning? The line is said by a character in anime. What is here (I assume it's somehow related to negative form of )? Plain negative (I'm not killing you)? Volitional neg. (I'm not intending to kill you (like the in ())? Potential (like in …)? Is the some auxiliary verb or part of conjugation? Something else?

(=) is the classical version of , 'do not'.

= the imperfective form () of the verb , 'do' ( = classical version of )
= the negative auxiliary << derived from the classical negative

( + particle (= here it can be like 'at least') + verb + negative ) is the classical way of saying , 'I'm not killing / I'm not going to kill (you/someone)'