I don't know of any toggle you can do. I checked in game for you in options and everywhere. the only thing i can tell you to do is to check your key binding stuff to make sure it looks like this

http://www.leagueoflegends.com/board...6&d=1268121950

Oh. About that. That's how Karthus' Q always works.

You didn't change any key bindings or anything like that. There are a few abilities in the game that always cast at/towards where your mouse pointer is. Lay Waste is one of them.

Another ability like this is the summoner spell Flash, which teleports you to the location of your cursor.

As far as I know there is no way to change this. You'll get used to it, and I find it allows for much faster more accurate spell casts with spam-able spells like Lay Waste.

It is automatically casted on your cursor when you press Q (or 1 or whatever).
This is probably because of the low cooldown of the spell.

The spell is set for auto-smart-casting and you cannot change it. (like Flash for example)

i hate auto smart cast so much it messes me up when im too far from the set point on a mouse and it goes some place else

Cassi's Q does this also.

When a skill has <2 sec CD, it MIGHT be a good idea to have auto-smartcast so you can spam it? Just get used to it, it's better than not.