I have always found Illusion to be extremely useful as an assassin. The three most important illusion spells in my opinion are Frenzy, Invisibility, and Muffle(this can be replaced if you have enchanted boots that have the Muffle effect). Frenzy will allow you to make your target attack others and if that target is weak, you have got yourself an easy kill. This is also useful against a huge crowd of bandits or mages. Muffle goes really well with the Sneak perks as you are virtually invisible in the shadows. Add Invisibility to that and you can virtually go anywhere in sneak mode without being detected at all. If you get your illusion skill high enough, you can take the perk that allows you to cast magic from any school without being detected. Again, this stacks really great with the Invisibility and Muffle spell. This goes very well in hand with Destruction as you can cast runes where your targets walk over them and hurt themselves badly in the process while you remain undetected, as long as you take both Sneak and Illusion perks. Dark Elf is the best race to start off in this direction as you have a +5 bonus for Illusion and Sneak and a +10 bonus for Destruction. You could also choose the Wood Elf, Argonian, or Khajiit as they are also great races for stealth type characters that don't use magic. In this case, Archery and One-Handed is definitely recommended.