In an ironic twist, the only movie slasher who has no real motive for killing is infamous for his motive being a lack of motive. That's Michael Myers, who kills people because dressing as a clown in the '70s brings out your inner darkness (not to be confused with Mike Myers, who beats jokes to death). Every other slasher has a clearly defined and terrible reason to murder. Jason, for instance, is upset that horny teens let him drown when he was but a young turd with nary a gleam of murder in his eye. Since then, he somehow grew older and became evil and repetitive. So evil and so repetitive. Freddy Krueger just wanted to be a pedophilic murderer but had his hopes and dreams cut short by the parents of his victims, so he came back from the dead for vengeance, in stark contrast to everything you understand about how vengeance is supposed to work. They may be dumb motives, but they are motives.
If you want to be a good slasher, where in this case "good" means memorable, prolific, or at least sequel-worthy, you can't let motive get too complicated or sensible. Look at Jigsaw. He's killed people across about 30 films, despite dying of cancer in the third film, and his motive may be the single stupidest motive in the history of film, horror or otherwise. He's sick, see, so as a man with a newfound respect for life, he wants others to be more respectful of their own lives, even if that means putting their heads in an electrified bear trap lined with shotguns and AIDS. Don't question anything he does or how he does it for a dozen films after he dies, because then you'll miss the clever death scenes, and that's how motive gets away from you.
So basically all you need is something that can loosely tie you to a victim, and the bigger the pool of victims, the better. Might I suggest you find a way to get horribly wronged by the fans of Game of Thrones, such that you need to stalk and murder them all with props from the show? If you can be the person who beats people to death with Peter Dinklage, that will be exceptionally memorable. Or maybe your family was killed by drunken cosplayers and now you're the Phantom of Comic-Con, strangling nerds with their own entrails. It could work. When all else fails, relate your heinous actions in some way to sex and sexuality. Like say you're a virgin and now you need to kill everyone who's ever touched a boob because something something your mother/rage/voices from the back of the toilet.