Given Lightning Blast, Shivan Meteor and 4-mana common removal spells like Grisly Spectacle, this could plausibly be an instant.

Yes, I would say this is fair when compared to Homing Lightning.

Could probably do more damage if you wanted it to... maybe six or seven (certainly not necessary, since not every common should be as efficient as it can be, but there wouldn't be anything wrong with it). It's actually kind of funny how direct damage to creatures scales, eh? Had you made a card that cost , I probably would have suggested dealing 10 damage at instant speed. There's a point where the red spell just says "kill target creature".

I think 5 is the perfect number her for the set, since the few 6/6 will be harder to take care of and will need more planning. I'm not a big fan of cards being able to kill anything for cheap costs if it's not black.