1. Both will go out of their way to defend themselves or their partner.
Couples and serial killers both cherish what they have. Whether that is the ability to kill people or to have the person you love by your side, protecting who what you love matters. Always take the time to help your s/o and show them your love. Stand up for them when they can't or shouldn't stand up for themselves. It is okay to be defenseless with your s/o, but be careful about how vulnerable you are. People tend to use this against you. Like we have our armed services to protect at domestically and foreign, we must also protect ourselves from the vices within others, while still seeing the good.
2. Both establish a common ground or "local haunts".
The place you always go where everyone knows your name. That movie theatre, restaurant, bar, bowling alley (I'm not judging), is your comfort zone. These are the places you know what to expect from. You are familiar with. Just like a serial killer. A serial killers comfort zone is where they are comfortable getting victims and killing them. Whereas a couples comfort zone may not be so dramatic as slashing someones throat, but it is doing what you are okay doing. There is nothing wrong with that. Sometimes though, it can get boring, routine. Try to switch it up every once in a while. "Go beyond what you see" and try somewhere new.
3. Both are willing to fight to continue what they have.
Both understand value. Nobody is better at looking into your s/o's eyes and knowing they love you. Being willing to fight against all likelihood that it will not continue because you cherish and value the one you have. Fighting to keep each other, fighting to keep loving each other. A serial killer will fight to do what he values- killing. This is of such great value to him that the costs become obsolete as he seeks after the thing that gives him value. Is this right? No. Augustine taught of ordered loves. Where desires should submit to loving the other things right first. Killing people is not showing love. Fighting to maintain your relationship, good.Killing, bad. Love, good. Got it?