Does Jourdonnais's ability negate all or half of Slab the Killer's attack on a successful draw?
Jourdonnais' ability is essentially a built in barrel.
Another Missed! card is needed in addition to drawing the Heart to avoid Slab the Killer's BANG!
This was answered by the designer, Emiliano Sciarra on Boardgamegeek:

Yes. Even if the effect of the Barrel triggers by drawing a Heart card from the deck, you will need another Missed! card from the hand to avoid the hit from Slab.
Since Jourdonnais has a Barrel built in, he may &quot;draw!&quot; once to see if he plays an automatic Missed!. After that, if he drew a Heart he will need a real Missed! from his hand to dodge Slab's shot. Otherwise, he will need two Missed! cards, as anyone else.