As an Assassin, you have three big concerns:
1. How do I study the target for 3 rounds and then get in close without being detected as a threat?
2. How do I make a successful hit when I attack?
3. How do I make my getaway if the target doesn't die? (Or if he does die.)
A Beguiler has lots of ways to do the first one. They can go the disguise route and pretend to be someone harmless, palming a dagger via slight of hand. Or they can use Invisibility and Zone of Silence to get in close unobserved.
Number 2 is tougher. On humanoid targets, you want to do everything you can to catch the target out of armor (and flatfooted). You can also get True Strike as a first level assassin, casting it silently with a lesser metamagic rod (or inside your zone of silence, but positioning is tricky).
Number 3 isn't purely a matter of mechanical builds, but of in-character planning and resourcefulness.