The High Aldwin
Sure, there are lots of builds that make good "assassins" without having the Assassin subclass. Of course, you could always dip into Rogue and take assassin as well, making you even more formidable.I just noticed that a Battlemaster fighter with Ambush, Precision and Menacing strike could go a long way to make an assassin.
Just take the Poisoner feat at 6th, Skulker that 14th and maybe Prodigy or Skill expert or Alert somewhere to have expertise in Stealth. Create a background with the feature of either charlatan or faceless.