I'd like to build a paladin but the group has a ne rogue assassin which prevents this from happening.
Why does this prevent you playing a Paladin?
First, just because you - the player - know that this other player's character is a neutral evil assassin, doesn't mean that your character knows.
And to be honest, if anybody in your group knows the character is an assassin, or the player (as their character) is telling the other characters they're an assassin, then they aren't a very good assassin. Besides, it's the responsibility of the assassin's player to maintain a reason to work with the group, rather than just kill them all.
Second, just because your Paladin is lawful good, doesn't mean they don't realize that evil does exists in the world, that evil is impossible to eliminate, and that it must be tolerated to a certain extent. Besides, actions speak louder than alignment. In other words, it's not necessary for the Paladin to have issues with the assassin character just because they are evil, but only if they observe the assassin performing evil acts. Even then, the world isn't so black and white. The Paladin may be black and white, but they know the world isn't.
Fiction has examples of Good and Evil - or Lawful and non-Lawful - coexisting and even working together (at least as far as fictional characters can be categorized by alignment - an usually tricky and ambiguous endeavor).
Think: Odo and Quark (Star Trek: Deep Space Nine)
Raylan Givens and Boyd Crowder (Justified)
Dumbledore and Snipe (Harry Potter)
Besides, you could always play it as your Paladin character is trying to convert the assassin character back to the side of Good - through example and mentorship rather than heavy handedness.
Remember that the Paladin's alignment and code is for the Paladin - not for the rest of the world. It's how the Paladin has chosen to live, and not necessarily a view that they're trying to force upon the world.
*And have some xp to get rid of that silver bar.
