I'm making my second-ever character in D&D 4e and I want him to be a not-so-good guy (my first character being a Dragonborn Fighter who worships Bahamut) that can do a ton of single-target damage. The options I'm considering are Rogue and Assassin, but I don't know enough yet to choose between them. I want to make an awesome killer who lies, steals, and – being an awesome killer – does a lot of damage! Am I better off with a Rogue or an Assassin?
Let's look at all 4 classes here at L1.
There are a number of different combos here, but let's do the basics:
First we're going to go with a dex of 20 for all of them, just to keep things nice and even. We will assume rogues/thieves takes the L1 Feat Light Blade Expertise. And assassins take Ki Focus Expertise for their L1
Dagger Rogue 8.47 DPR
At Wills: Sly Flourish (take Cha 16).
If we can assume CA (not a given) here your attacking at +12 with a damage of 1d4+9+2d6 for 16.05 DPR. CA cannot really be assumed this early, so typically you're going to be attacking at +10 vs AC with damage at 1d4+8 so 8.47. The plus here is that this works at both melee and range, where other options are melee only.
Charger Thief 17.08 DPR
With the charger thief we can take a background to get a rapier at L1 and still have the feat slot open for LBE. After that we take a normal charger thief build (tactical trick and ambush trick combine for continuous CA on your turn as long as your aren't action limited). This gives you a +12 to attack with CA, with damage of 1d8+7+2d6 for 17.08 DPR. Advantage: is Melee basic attack driven so it operates nicely with a warlord, bard or artificer, with the caveat being he needs CA some other way when it's not his turn.
Normal Assassin 9.4 DPR
We'll use Shadow Storm (gosh those at wills are...underwhelming IMO), and assume we invoke one shroud/turn so we get +9 to hit with damage at 1d8+5+1d6. So DPR is 9.4.
Exe Assassin 11.55 DPR
There are several schools here, but for this none of the matter, we're building a charger with a rapier. Charging we get a +10 to hit, and damage of 1d8 + 5 + 1d8 for a DPR of 11.55. This build has the advantage of providing the most reliable basic attack driven DPR (the extra damage is no dependent on CA or it being your turn.) The only catch here is that there aren't many options to charge out of turn so your DPR will be a touch lower.
Which one of these you pick is largely situational. The clear loser is the straight up assassin, but I can safely recommend either of the other three options with the caveat that for a starting player the Thief and Executioner are much easier to build and play well than the Scoundrel Rogue is.
The difference is:
Do you want to stealth in, assassinate someone, and stealth out? Or do you want to function in the traditional dungeon environment?
The assassin, especially the executioner (my taste is for the ninja subclass...) is slightly better at the first case, but less generally adapted to the dungeon crawling environment than the thief. If you want to be a lying, cheating, killer, my recommendation would be for Changeling Thief, simply because the face-changing provides a very novel mode of social interaction that really does synergize well with bluffing.
In terms of damage, the charging thief is very hard to beat, with a correctly built unseelie agent thief killing, on average, .86 standard monsters per round. (contrast with the fighter who aims to kill .25 standard monsters per round as a high standard of excellence).