Batman: Arkham Origins

Alfred: Do try to be careful out there sir.
Batman: Don't worry. I'll take out those assassins before they know I'm there.
Alfred: It's not just that sir. If a curious child wakes up to see who's pattering across their rooftop, they'll expect Santa Claus. Not a black-clad bat creature.
Batman: I don't patter.
Alfred: *Sigh*.

Question: When Batman and Alfred look at the memory card and come across Slade Wilson, Batman says that Slade's part of a failed experiment. In what way was the experiment a failure considering how efficient a killer Slade is?