Murder mysteries always have one guaranteed plot point in common: someone in the book has killed or will kill someone else. We might not know why at the beginning of the book, we still might be unsure by the middle of the book, but by the end of the book, the protagonist will discover the motive behind those killings.
But how serious does the motive need to be? How cranky will the reader be if the killer has a weak motive? Certain motives are always accepted. A killer might be compelled to murder to avenge an injustice (real or perceived) against themselves or loved ones. People often kill for financial gain or to prevent someone else from revealing their evil doings.
And then there are murkier motives. Like a guy who kills the person that accidentally ran over his pet turtle (okay, I made that up). A few months ago, I read a story where the killer acted to hide a scandal that involved their great grandmother, who had been dead for decades. Sorry, that's not a valid reason in my mind to kill someone. The lack of compelling motive ruined the entire book. Made it all seem so silly.
The motive must be even stronger when dealing with a serial killer. Sure, when a person kills multiple people, they're generally insane, but they still need a good motive, regardless of whether it's based on real events or those fabricated in their head. And the more people they kill, the better the motive needs to be.
The best motive is one where you almost feel bad for the killer. You can understand their reasoning and wonder if you'd respond in the same manner yourself. Because no one is all good or all bad. Everyone has varying shades of evil in their personality, which makes picking a motive that much easier.