Going for LE assassin, start with rogue or ninja?

[MonkeySage]

I'm thinking of playing as an LE halfling hitman. He's in it for the money, not for revenge, fanaticism, or sadism. He abides by a strict personal code but actually, while he does try to get the law on his side, it's more out of convenience than out a genuine respect for the law itself.

He doesn't actually work for any organization (yet).

Both Rogue and Ninja have their benefits and drawbacks, but I don't personally see ninja as the sort to seek work as hitmen.

[Andry]

Or here are couple of other options you could go with Slayer or a Vivisectionist Alchemist.

[Zhentarim]

Or here are couple of other options you could go with Slayer or a Vivisectionist Alchemist.

I second this. You can be a slayer vmc rogue and be just as good at sneak attack as a rogue and have full BAB. Plus, slayers get enough bonus feats to almost cancel out the loss of feats from VMC.

An interesting choice for a future assassin is to be an inquisitor (sancitified slayer) of a lawful evil diety and also take vmc rogue. It will give you a leg to stand on if you need a place your your character to hide out a while without bluffing (ie: you go to a temple of asmodeus in cheliax and you get free room and board) and you can heal yourself if you get a boo boo assassinating people.

[Particle_Man]

Unchained Rogue is nice for a little extra damage from dexterity and you might find a skill unlock you like. Plus skill points!

If disguising your alignment or job is important, bard can be a nice cover and they can use a spell to hide their alignment.

[Geddy2112]

The cool thing about assassin is that basically any class can become one. The thing is, pathfinder is a combat heavy game, and every class has mechanics that focus on killing things. Most campaigns are the adventure chronicles of a group of murderhobos. So play what you want and what fits the character, then throw assassin on top.

Since the assassin's death attack is the main reason to become one, you probably want to have decent INT to make sure the death attacks stick. Ninja is stronger than rogue but CHA based, making it a bit more MAD of a combo. Having an in class ability to turn invisible is really good when your death attack relies on not being spotted. Vivisectionist is INT based with in class invisibility as well, making it one of the stronger contenders for assassin. Have you considered investigator as well?

If you do go rogue, consider unchained rogue as it is basically a straight upgrade from regular rogue. There is also unchained rules for the ninja as well.