Is assert evil?

No, neither goto nor assert are evil. But both can be misused.

Assert is for sanity checks. Things that should kill the program if they are not correct. Not for validation or as a replacement for error handling.

No, there's nothing wrong with assert as long as you use it as intended.

That is, it's supposed to be for catching cases that "can't happen", during debugging, as opposed to normal error handling.

• Assert: A failure in the program's logic itself.
• Error Handling: An erroneous input or system state not due to a bug in the program.