Dire Bare
Legend
Yes, many parties are morally ambiguous, but an assassin is a lot more so. But the main question is what kind of assassin. If they kill for their own monetary gain then they are not heroic. If they kill for a cause they can be. If they kill to make the world a better place, then they probably are.
But for many people heroic is facing enemies in the proper way, and not using any means necessary, and that is the rub for me. I want my players to be heroic, I like to run heroic games. And a lone assassin who sneaks in and out of places just is not.
Also D&D is ever a party game, and assassins are not party people. Yes, they can play as part of a party, but then they are not being true assassins. They could be any of a number of other classes just as well.
I have not seen the new assassin in 4E (see my sig) and probably would not object to someone playing one (assuming I thought he was balanced) if I had the rules for him, but they would need to fit in the party.
So a lone killer would not make the grade.
You're making a lot of assumptions on how assassins "must" be played. Sure, the NPC assassin guild is filled with amoral loners who enjoy their work a little too much . . . but that doesn't mean your PC has to fit that mold.
It doesn't take much thought, IMO, to create as assassin who can be a hero (if a dark one) and a team player. Perhaps a religious guild of assassins dedicated to a good or unaligned deity that target those who do great evil and oppress the weak. Perhaps your assassin IS quite mercenary, but when it comes to his friends he is loyal and true and when the entire world is in danger from demonic invasion (or whatever great evil the party is facing this week) he's more than willing to risk his own life for the greater good.
Novels, comics, TV, and movies are just FILLED with dark heros and even villians who fight alongside the heroes when the stakes are high. Why do so many think this is not possible in a D&D game?
If you still just can't see an assassin who's still "on the job" as a part of a heroic party . . . have your PC be a former assassin (but still of the assassin class and skill set) who now fights the good fight. Hounded by his former comrades because he broke ranks, perhaps even former friends.
I have played with some jerks in the past who somehow thought playing an assassin (or other dark character) gave them a license to screw with the party and kill random NPCs . . . that I certainly won't ever tolerate again!