It is certainly possible, you can create pathological regex that have exponential time requirements.
Probably the solution here is to forcibly turn off edit mode so Keyboard Maestro editor probably wont try to evaluate the regex when you launch it. Run the Terminal command:
defaults write com.stairways.keyboardmaestro.editor MacroWindowEditState -bool NO
Then launch the Keyboard Maestro editor. Hopefully it will launch without freezing. Then you can delete the macro in question. If the macro is complex and you just want to delete a single action, that would be more challenging, but you can probably export the macro, delete the macro, hand edit the XML of the export to remove the pathological regex, and then re-import the macro.