If the target of a spell dies before it's cast I think it should shift to a different mob in the fight not just cancel and do nothing. Especially when it's a multitarget spell i.e Multiblade. As it stands it's basically just a waste of a turn.
There's been some discussion from the player base trying to get this spell to not require a target. The workaround for now is to target yourself with it and it will cast.
Pro Tip: When multi-blading, if you target yourself, you wont encounter this problem