Guilt and mens rea can be quite compatible with your admittedly strange idea of there being no free will (and yet trying to parse laws under a framework of people having free will), unless you believe that all acts are coercive (which is quite reductive).
All you need to ask yourself is if the person wanted and intended to do that, whatever the nexus of causes led up to them wanting to do the act.
It seems very weird (and a bit lazy) to subscribe to a framework of there being no free will and yet not even trying to contextualise the safeguards of the legal system to fit that framework. Sure you may agree with putting people in jail to prevent net societal harm, but mens rea is one of the checks to ensure that they will cause societal harm to others, and without being able to settle such a question of fact you will instead never be able to put anybody, even if they need to be put behind bars, there.
Guilt and mens rea can be quite compatible with your admittedly strange idea of there being no free will (and yet trying to parse laws under a framework of people having free will), unless you believe that all acts are coercive (which is quite reductive).
All you need to ask yourself is if the person wanted and intended to do that, whatever the nexus of causes led up to them wanting to do the act.
It seems very weird (and a bit lazy) to subscribe to a framework of there being no free will and yet not even trying to contextualise the safeguards of the legal system to fit that framework. Sure you may agree with putting people in jail to prevent net societal harm, but mens rea is one of the checks to ensure that they will cause societal harm to others, and without being able to settle such a question of fact you will instead never be able to put anybody, even if they need to be put behind bars, there.