What makes a killer? Killers: is maternal deprivation to blame? Institutionalisation and murderers. Does attachment type influence criminality? - all psychological questions.
Killer: material deprivation, cultural deprivation? Do serial killers act out of frustration towards society? Is relative deprivation to blame? - all sociological questions.
Also, why wouldn't you be allowed to do nature vs nurture, I think that's perfect? In fact when I did my EPQ I was advised to do something related to my subjects, which coincidentally were biology psychology and sociology like you. It will look really good with unis when you apply.
Seriously, link it to both psych and soc, it will lead to easier research thanks to your textbooks and will look very academic if you choose to do a dissertation.
P.S your EPQ teacher does sound very
S H I T
I've never heard anybody conduct an EPQ like that.