http://i.imgur.com/0h9FZ7U.png here's a quick chart of some of marth's moves, it displays knockback and knockback growth.
although some of these have high knockback, few are reliable or useful as kill options, however.
since marth's untippered f-smash has a terrible angle and knockback-growth, as well as being a risky move on the whole, its animation being 3/4's of a second with only 4 active frames, it is a surprisingly situational move.
try dancing blade(first hit only) into a tippered
up-tilt at 130%+ versus peach, marth, and sheik. uptilt tippered will also kill on pretty much everyone but falcon on pokemon stadium at 140%+. you can also score kills or edgegaurd scenarios when you get the elbow hitboxes against bad DI (elbow hitboxes being the two innermost hitboxes), especially versus spacies, since it launches at a 45 degree angle, meanwhile they were expecting to be hit straight up or diagonally up.
D-smash gets some honorable mentions because it actually comes out waaay faster than f-smash. its active by frame 5, and will likely connect twice as fast as f-smash(less reach but no less disjointed), and is even faster than d-tilt. its weaknesses being that the untippered dismash hits will almost never kill, its your laggiest move, the hitboxes only cover a small area, and its setups are extremely limited. you shouldn't pull this out more than once per game at most, but you can use it to kill floaties off of a tech(or missed tech, it comes out faster than grab, so go nuts) and maybe as a shield poke. a tipper dtilt pretty much hits the same place as tipper dsmash. not exactly a 'good idea' or anything, but dont forget it exists.
Up-special sweet-spotted and reverse sweet-spot can be combo'd into, performed off-stage, and comes out lightning fast, and is your fastest option out-of-shield(even faster than grab, done frame perfectly). i haven't fully incorporated this into my out-of-shield game because its booty anyway, but being able to punish out of shield with a kill at high% is a godsend for any developing marth. i highly recommend working this into your gameplay. will not kill until well over 100%, especially versus floaties.
some honorable mentions that probably wont kill people but have more kill potential than f-air:
- n-air: on account of having a better angle and slightly more damage than f-air
- tipper b-air: same reasons but also covers a huge area
- f-tilt: everyone already said f-tilt but i'll say it again. tipper ftilt is good, its knockback is lower than other killmoves, but has a more lethal angle, and sweeps the area at marth's feet with its sweetspot, making it very likely to tipper at the ledge.
- counter vs. falco: you can counter his firebird at the ledge and if he used his jump, grab the ledge and he's dead at most %. if he hasn't used his jump, he will need to be at least 50% for this to kill him, probably more like 70% but im estimating. i put this in because its easy, and because falco is the only character who could ever really die from this. i do not recommend using this unless its on reaction to his angle. this doesnt work against side-b unless its a particularly scrubby one, because the hitboxes on illusion travel in falco's wake, rather than ahead of him, thus it wont work unless his side-b ends right in front of your face.
this is way too long so tl;dr, dacning blade into utilt, up-b/reverse up-b sweetspot is sick out-of-shield or in the modified ken-combo, everything is better than f-air at killing.