Is Killer Sudoku harder than regular Sudoku?
The answer depends entirely on the difficulty level. Easy Killer Sudoku is often simpler than equivalent classic Sudoku. Expert Killer, with no given digits whatsoever, is considerably harder than classic Sudoku at any level.
Why Easy Killer Can Be Easier
In classic Sudoku, given digits tell you nothing about nearby cells - Each one is an isolated fact. In Killer Sudoku, a cage with only one possible combination (such as a 2-cell cage summing to 3, which must be {1,2}) immediately gives you two cells worth of candidate information. Small forced cages act like given digits and sometimes provide more information than a sparse classic grid at Easy difficulty.
Why Hard Killer Is Much Harder
At higher difficulties Killer Sudoku removes all given digits entirely. The only starting information is cage sums and positions. You must apply the 45 rule, find innies and outies, and use cage interaction techniques before any standard Sudoku methods become available. The extra layer of deduction increases both complexity and solving time significantly.
Technique Requirement by Level
Easy Killer: Forced cages + 45 rule + basic scanning.
Medium: Multi-cell innies/outies and cage overlap.
Hard and Expert: Complex cage interactions, multi-region 45 rule, and full standard Sudoku techniques on top of all the above.
The total technique burden at Expert Killer exceeds Expert classic Sudoku for most solvers.