Mysteries abound. And, if you read them all, you'll find that there are lots of different reasons why someone might want to commit a crime. Money, love, anger, revenge, mental illness, etc. So many reasons to do bad things.
If you're writing a mystery, there are likely going to be many suspects to the crime that happened or the threat that permeates your story. You want every one of those suspects to have a reason that will convince your readers that they could be the villain of your writing.
One thing to remember is that they don't all have to have the same motive.
Even if many characters are accused of killing the same victim, they don't all have to have the same reason. Someone wants an inheritance. Someone was wronged. Someone else wants to take over the business.
There are a lot of reasons to kill. Give every one of your suspects a motivation that is true to them.