If guilty one of the biggest hurdles is getting people to believe someone can look normal, yet commit multiple murders. Due to media depictions, some people still expect a swivel eyed, trench coat wearing, knife wielder.
In truth some serial killers can present quite normally, be married and have families. They manage to commit multiple crimes undetected, sometimes over years or decades. They don’t present as an obvious danger, and can be highly convincing/ charming, even with law enforcement. Some sk have multiple police interviews and are ruled out as suspects only to later be convicted.