This is a very odd question, but the personal reasons are not sufficient to warrant legal action and/or renege on a contract. Most people are disgusted and offended by sex offenders, but the house is just a house. Are you being paranoid? You can have it checked for wiring and any cameras secreted around the house. If something is indeed found, which is dubious, you may be able to take the seller to court and/or have the person responsible arrested.
If this person was arrested for sex offenses involving cameras or voyeurism, the police would have thoroughly scoured the house while investigating and gathering evidence.
Inspect the house for wiring and any cameras hidden in places such as smoke detectors, outlets, or appliances left behind. Cameras can be concealed inside ventilators, vents, microwave glass, even in television screens. But a thorough inspection should assure you that no such cameras exist. If you really want to be certain, hire a private firm which specializes in surveillance. They will be certain to clean up the house of any cameras or microphones, etc.
In my town, a man was arrested recently for renting out rooms in his house and spying on female tenants; there was a recent news story about a group of men, apparently copycats with internet contacts, planting what looked like ordinary phone chargers into outlets in female Starbucks bathrooms, with a remote WiFi connection for tiny cameras inside them; something like 21 of these freaks were arrested by the FBI in a nationwide sweep.