As much as I'd like to believe Light Yagami would be the dreamiest guy to be in a relationship with, he wouldn't.
Before that, let's compare him before the death note.
Obviously, Light would probably be a pretty good boyfriend.
He’s smart, responsible, and calm. He’d help you study, walk you home, remember important things, and give solid advice when you’re stressed. Your family would adore him.
But even then, Light would need to feel admired. He’d want to be the smartest one in the relationship. He’d want to feel respected. Worshiped in ways.
He could love someone, but he’d still need to feel superior. Which could cause major relationship issues in the future.
The death note didn’t create his ego.
Now, obviously his treatment of Misa proves that he had grown to be more toxic after the death note, but he didn't really love her. What if he actually loved someone? Could he throw his ego to the side for them?
Because Light doesn’t just have an ego, it's basically who he is. His identity depends on being the smartest person in the room, the one in control, the one who is better than everyone else. Love requires vulnerability, equality, and the willingness to admit to wrongdoing.
Light would rather lose a person than lose control.
What if that person questioned him, even about the smallest thing, would he listen? Or would he see it as a threat?
Light doesn’t handle threats with emotion or communication, he handles them by getting rid of them. By nearly any means necessary.
Even if he felt something genuine, the second that relationship endangered his image, his plan, or his god complex, he would choose himself. In his mind, he is saving the world. Any personal attachment that interferes with that becomes dangerous to him.
The scariest part is that he’d probably convince himself it was justified, and that you would be in the wrong. Maybe even manipulate you into feeling guilty.