Consent and serial killers aren't mutually exclusive, it's not uncommon for a serial killer to ask for benign consent of something small and take it as permission to do something awful that obviously exceeds the previous consent given, the 'they asked for it' effect. Added onto that is the fact that I'm stating it was implied, not empirical proof, Sherlock.
What matters is you're going around asking for skin to do experiments with and you're not telling people why. That's what matters. Why would you need that for anything other than chemical reaction or decomposition study?