You can kill if you hit them with a fair offstage or near the edge, it's pretty strong.
Fsmash and Dsmash are the moves you should save (you can use them at the beginning of the stock once each, but after that you should save 'em).
If you're having a lot of trouble killing until high %, look for gimp opportunities. Snake's recovery is very linear and I believe you can Diddy Hump him. You can also Dair spike him.
D3 is a little harder but you can get him to high %'s fairly fast because he's easy to combo. Glide tosses and tilts just **** him. Use throws to put him off stage and force him to use his Up b to recover if you can (it's easy to punish). He is a big target so your rocket barrel spike is a viable option. Just be aware that his recovery does go pretty far so at low % this might not kill him.
Uair can kill sometimes if you nail your opponents a considerable height above the ground. Dash attack combos into uair, fair and dair, so you can set up for KOs fairly easily with this. If you knock them past the edge with the dash attack, use a dair (see NL's signature), if they're not quite there, a fair will still do good, and if you're more centralized then a uair is good. Even if it doesn't kill it's good juggling.