Hearthpwn

For me, the art looks like an ogre is in a guillotine and some other ogre is torturing him by pouring poison on his neck.

About the flavor text of this card...I want a Poison Elemental.

Poor Rogues 

if you can equip a +3 atk weapon as a warrior why, isn't this at 0 mana cost too?

This is much more flexible than Fiery War Axe. You can increase the attack of your other weapons too.

I think this card would easily be fair at 0 mana.

I would argue that this card's nearest analogue is Totemic Might.

1) Both cards have a prerequisite to cast. (which can be a big deal in a game with only 30 card slots)

2) Both cards have synergy with their class's hero power, but there are also other class cards that they can be used on for better value.

Shield Slam might also be considered an analogue for these reasons, though I think it's cost could be justified by the fact that it deals direct damage.

Totemic Might gives health to minions without attack damage (most times) while Deadly Poison gives you (almost) direct damage that has instant impact on the board.

Also Rogue has a mechanic , Combo , which lets Rogue to gain more value out of her cards that also synergizes with cheap cards.

Also since Blade Flurry is bullshit , its best to keep this at one mana. 

A weapon buff is not the same thing as direct damage (or almost-direct damage). If we imagine Deadly Poison's damage output to look like this: 2 damage * weapon durability, then we can imagine a version of this spell that does do direct damage.

1 mana - deal 2 damage for each durability your weapon has.

See the difference? Imagine a scenario were you want to kill a Boulderfist Ogre and you have an Assassin's Blade and a Deadly Poison. Our fake version will kill the ogre, but the real version will not. How about a scenario were you swing away the last durability of one weapon, then equip another? Our fake version will have an effect immediately, the real one won't. 

Funny thing is, our hypothetical card reminds me of a real one: Shield Slam. Once upon a time Shield Slam was nerfed to only be able to target minions, so lets just assume that the same thing would happen to our fake card (no big deal, right?). Now we have two cards that are essentially one-mana conditional damage. But that would mean our fake card is appropriately costed! So what does that make Deadly Poison? Over costed?

Also, Shouldn't there be cards that synergize with Combo? 

One last thing, Blade Flurry is a harder card to full value out of then you might think. If you try to get full value out of a big weapon like a buffed Assassin's Blade, then you become vulnerable to weapon removal. And if you want to play a bunch of buffs on a Wicked Knife then use it, then that's a combo that requires some planning ahead...

As this poison is glowing, its probably jam-packed with radium so if the knife doesn't kill you, you get leukemia a few years later. How cruel!

Is there any type of poison that isn't deadly?

Quote from SillySocks »

Is there any type of poison that isn't deadly?

Yes poison can be classified as a substance that just causes injury or illness, if you don't want to kill someone.

Immobilization poison.